There was a chap on the Flyer forums (SteveR) who used to keep his Gardan Horizon at Milborne St. Andrew....I'll drop him a PM. It'd be pretty handy for me too - I'm at Henstridge at the moment but live on the coast...
I tried the gliding site at Bovington, but it appears there is no chance....
Edit: His username seems to have been deleted from Flyer so not sure what happened to him...?