HD,
re jumpseaters point of trying another body / lens - I would be happy for you to use mine when you next visit Popham (8th October is final event of season)
Having 1 or 2 pictures on airliners.net / jetphotos net it can take a while to get an image up to their standard but judging by the queue length on both sites enough people manage. It is also possible that you do have a fault with either camera or lens - my 20D body suffered a partial autofocus failure at RIAT in 2005 which was very frustrating but it worked enough of the time to get some shots.
rgds
Paulc