When did things start going wrong for you in the race Mark? Are we talking hitting the wall or just running too slowly for most of the race? You are clearly covering the right kind of distances in your training runs if you're doing 21 mile long runs but anything can happen after 22/23 miles even to those who think they're prepared. How much mileage were you doing during the week? Did you taper properly?
Would echo what others have said about race weekend preparation. I do as little as possible the day before big races (other than a 3 mile jog). In fact i go to extreme of taking Thursday and Friday off work when I have a marathon. You won't have been the only one affected by the sun as although it wasn't drastically hot compared to some years the sun certainly seemed pretty strong.
Personally I would definitely wait until October as a marathon takes a lot out of anyone especially if you've got seriously dehydrated. Regroup and write your training plan for York (and make sure you include not over exerting yourself in the few days before!). Minni may have a good point with getting a good base in.