Tay, indeed. I got a number of Spitfires and they all came free but with full fuel on arrival for the display and on departure. I can say this now, but Nick Grace flying from Southampton said he would be out of pocket using all his 'free' fuel to get home again. He asked and we gave him is fee - £190!
G-FIRE OTOH was transiting north so was able to come to us on minimums and continue north on free fuel, win-win.
Of course free fuel only works if you have Avgas. For one participant MT had to take a Jerry Can to the local garage for some low octane stuff. I guess unleaded has grounded him.
It also depends on the show. I am at the wedding/barmitzvah/village fete level rather than RIAT but to be invited to one of the bigger shows would probably result in a lot more flexibility of charging from my point of view. Ahoy, RIAT - I'll do it for fuel only!
