+1 to the MSeries.
The week before last on TRAT, I learned that when it's stinking hot, drinking a bottle an hour is pretty much a prerequisite for me not conking out. I made up my bottles with one containing strong Ribena and the other with Boots rehydration salts. Combined with enough food, if I managed to do both bottles in the space of two hours, I didn't feel weak, dizzy, crampy or any of the other symptoms I've felt in the past.
I shall probably run on coke + water or whatever I can buy in garages this weekend.
Other than that, wear sunscreen. It helps.