I called in to the East Somerset Railway. That are happy for MH to stay for £7 a night even when closed on Mondays. It's a worthy cause and i would find it an interesting place to stay not just a car park. I sent the usual campra links. Not sure yet of facilities but you would think a steam railway would have taps (and coal) at least. I'm not local but if anyone was interested in face to face feel free. Pub nearby. They are very responsive and easy going on Messenger.
Details on Searchforsites