Penguina,
I take it from your post that the Sunday may not be the best day to go to the Show if the London Marathon is on that day - I for one had no idea this was the case. I seem to remember Marathons being in the order of 27 miles.... as one that does not live there, is that quite alot of London to close off? I could well switch to the Saturday 17th now, if I go.
I wish you luck with the Marathon Run,
Regards, SD..