Julian,
the way around this thorny problem is........ to buy the whole site yourself.
It's a well known fact that Peel only took on responsibility for the site with the eventual aim of developing Finningly, Peel also made no secret of the fact that as soon as Finningly was up and running commercially they would dispose of Sheffiled.
Most of the money for the development of the airport in it's infancy came from Budge Mining (around 10%) and the EEC (around 89%), the rate payers of Sheffield barely contributed anything through the local authority, other than relatively insignificant sums via the Regional Development Agency.
After that, Peel put in around £8m to keep the place going, so it's only right that they should have a return on their investment for providing the community with a facility that the community never paid for.
I accept that you and your colleagues pay to be based there, but it's no different from renting a commercial premesis, once the landlord serves notice, you are obliged to go and your opinion counts for little unless you are prepared to make a vaible commercial proposition.
The other thing you should consider is, that the airport is built on a slag heap, and I don't know if many people are aware but it only ever had a fixed life span because of that, many of the existing buildings regularly show signs of subsidence and the runway probably only has a few years left in it.
As an industrial estate, it has a future and would be a far greater boost to the local economy than the airport ever was, the subsidence problems wouldn't be too much of a factor - as an airfield it has no commercial future, doesn't serve the local community's needs at all and never will.