Doppleganger,
A misunderstanding. It was not apparent that you were attending a free-flight day, and I had visions of some idiot driving a 1/4 scale Lancaster into the crowd and thinking it was great fun.
Unfortunately the hobby has a lunatic fringe who have no regard for safety, insurance or anything ( or anyone ) else. They can hardly be rated as modellers - they just buy big boy-toys, and buy another when they wreck the first. Clearly you are not one of them and I apologize for lumping you in with them
I cannot think that the person checking BMFA membership could be the threatening letter writer. He would hardly have approved of the BMFA. I am surprised that you have not heard from the Trust - be interesting to know if they do check up on insurance or if it was an over officious BMFA member.
Good model flying,
O Xenos