It was the late 1970s and a young museum officer from Shropshire council had stepped into his local for a drink after work. He found the atmosphere unexpectedly lively.

Centre of the merriment was a group of young curators who were laughing and joking at some volume. Keen to know the cause of their celebration, the man inquired and was told: 'We've just captured a chemist's shop in Bridgnorth.'

It sounded as though the gang had just hijacked the local branch of Boots. In fact, the shop, complete with all fittings and fixtures, was the latest acquisition for Ironbridge Gorge Museums. It would probably otherwise have been dismantled and the contents dispersed or dumped had the team not seized it.

The young council officer, Sam Mullins, was struck by the curators'
gung-ho approach and its contrast with the sedate world of museums he knew.

'I was in awe of what Ironbridge was doing,' says Mullins, who is now the director of London's Transport Museum. 'I was hugely impressed by the notion the independents had of capturing, rather than collecting, which seemed much more proactive. They talked as if it were big-game hunting.'

A pioneering spirit, a can-do attitude and an innovative, entrepreneurial approach were all hallmarks of the new breed of independent museums that emerged in that period. As well as Ironbridge, which is now a collection of ten museums, they included the Gladstone Working Pottery Museum in Stoke-on-Trent, the Weald and Downland Open Air Museum in West Sussex, the National Motor Museum at Beaulieu in Hampshire and the Bass Museum at Burton upon Trent.

Established by community or industry groups and focusing largely on transport, industrial and rural heritage, they operated as charities, raising their own revenue from admission fees and commercial activities and effectively free from the constraints of political control.

In 1977 the young guns leading them joined forces to form the Association of Independent Museums (AIM). Diana Zeuner has been the editor of the AIM Bulletin, the association's newsletter, for 28 years and her late husband Chris was the director at Weald and Downland.

She recalls it as an exhilarating period when the 'dull and dusty' image projected by local authority museums was swept aside: 'They were terribly optimistic, positive sort of people, with ideas and projects that were developed very fast. The directors said, "We are different so we should get together as a group and pursue our interests nationally and internationally".'

Neil Cossons, the former director of the Science Museum and the current chairman of English Heritage, was Ironbridge's first director in 1971 and became AIM's first chairman. He says there was a strong sense of a mould being broken: 'I had joined local authority museums in 1961.

The idea that this new breed of museums might be a way ahead for all the things that local authority museums weren't doing was exciting. They were dealing with collections and themes outside the traditional multidisciplinary role of a council museum. Local authorities didn't do industrial archaeology or trams. The independents were into new subject areas and single themes. There was a really good spirit of looking forward to a new future.'

It made for lively gatherings. Cossons says: 'If you went to Museums Association meetings at the time, people were wringing their hands about how miserable life was because of unpleasant councillors or budget cuts. At AIM meetings you could hardly hear yourself speak there was so much noisy chat and enthusiasm.'

There was, Cossons adds, some antagonism from council curators. 'There was a belief that museums were services that had to be provided by local authorities and if there were things that the authorities weren't doing, then they shouldn't be done.'

Perhaps surprisingly, Arthur Drew, the then chairman of the Standing Commission on Museums and Galleries (the forerunner to the Museums, Libraries and Archives Council - MLA) was supportive of AIM.

He described independents as the 'primordial slime' of the museum world, says Zeuner: 'Local authorities agreed because they could identify us as "slime", but AIM was very happy to be thought of as "primordial" - the origin of things, from which other things grow.'

In its early days, AIM was essentially a self-help group. It focused on peer support and sharing information about the business of being an operating charity in a world that was set up for giving charities. Membership rose steadily hitting a peak just short of 900 in the late 1980s.

As competition for visitors became more intense in the 1990s - a result of the growth in visitor attractions and, in particular, Sunday trading - some independents were forced to close and membership fell back. Today it stands at just under 800.

Members range in size from the big beasts of Beamish and Ironbridge to the tiny garden-shed operations, but small to medium-sized museums, initiated by communities and embracing all subjects within a locality, make up the bulk.

One defining characteristic of independent museums is their entrepreneurialism. It has been one of their key influences on the wider museum world, says Mullins, a former AIM chairman.

'Having a charitable constitution gives you extraordinarily strong purpose for what you are doing. What that did for the museum world was to introduce entrepreneurial attitudes in the sense of being socially entrepreneurial - how to fund preservation of significant bits of the heritage from admission fees. It meant that a commercial attitude - running shops, events, cafes and visitor attractions - was introduced to the museum gene pool. It certainly wasn't there before.'

Jonathan Bryant, another former chairman, points to AIM's success in promoting the advantages of 'clear and single-minded governance'. He says: 'One of the big difficulties for local authority museums is that they are in large departments. Local authorities have to make decisions not necessarily in the interests of the museum but of the authority, whereas an independent board of management can have only the interests of the organisation at heart.'

In the early years advocacy was not a high priority but that changed in the late 1980s and 1990s when AIM members were drawn into national funding schemes and a new world of regulation.

Mullins says: 'We found ourselves in a situation where regulation and accountability began to impinge on our world as it did local authorities - health and safety and so on. Local authority grant funding was also ebbing away from many independents and the Designation and then Renaissance schemes were used as replacements. That inevitably brings the independent sector into dealing with government.'

It forced AIM to expand its lobbying activities and opinion varies as to how successful it has been on this front. Mullins, Bryant and current chairman Bill Ferris, who is the chief executive of Historic Dockyard Chatham, believe AIM punches above its weight with central government and its views are taken seriously.

Mullins points to its role in the successful campaign in 2004 to prevent the Treasury abolishing museums' right to Gift Aid (whereby income from admission charges is treated as donations and the tax reclaimed).

Abolition would have deprived independent museums of a significant chunk of income, up to £1m at some larger institutions. The Treasury was persuaded to change its mind and a deal was negotiated allowing museums to hang on to Gift Aid, though with new conditions attached.

Zeuner is more circumspect. The way funding was linked to government priorities after 1997 did AIM members no favours, she suggests.

'AIM was very influential until Labour came into power when a new political agenda was set,' Zeuner says. 'The old Museums and Galleries Commission was very positive and in favour of independents and AIM had influence. When Labour took over, it was very keen on its particular political agenda of making museums free which, of course, the independents couldn't do.

The whole emphasis shifted to improving local authority and national museums, which is where Renaissance came through. AIM had to fight very hard to get a slice of that cake. We do know that there are more benefits to independents than it might appear and there's still a lot more that could be done to make Renaissance money go further.'

AIM has voiced its concern to the MLA at the way Renaissance cash is distributed. Ferris says the scheme fails to offer anything to museums that are already of a high standard.

'Museums like the ss Great Britain, which won the Gulbenkian Prize for best UK museum in 2006, are not benefiting from Renaissance because they don't need the kind of support that's currently on offer from the Museum Development Fund and development officers. Similarly, there are a lot of local authority museums in the South East that are very good but are not getting anything out of Renaissance at all.'

The other major worry is the state of local authority spending or lack of it. It's not simply a question of fewer grants as local authorities concentrate their resources on statutory services.

'It's actually things like closing the tourist information office or making the tourism officer redundant that are hitting us,' Ferris says. 'Where marketing is done in partnership with the local authority and then the tourism office is closed, it hits us hard because we can't afford to do publicity by ourselves.'

AIM will publish its own research into the impact of local authority cuts at its 30th anniversary conference this month at Ironbridge. It will also be unveiling a partnership with a new donor, which will add to an existing grant scheme for economic sustainability projects funded by the Esmée Fairbairn Foundation, which is worth £250,000 over three years.

The trust model popularised by independents is now heading for the mainstream as a growing number of local authorities explore the advantages of converting their museum services to (semi) independent charities.

Local authority curators in the 1970s who regarded independents, with their admission charges and freedom from council control, as a capitalist blot on the cultural landscape would be aghast. But Cossons says it shows how influential the independent sector has been.

'It is fascinating that places like Sheffield and Glasgow should be going down the trust route. It demonstrates that things have moved quite a long way in the local authority political field when that sort of change can be contemplated.'

Julie Nightingale is a freelance journalist

The 30th AIM annual conference is at Ironbridge Gorge Museums, Shropshire, 21-23 June. Culture minister David Lammy will give the keynote speech.