Thanks for the info Steve, I had a good breakfast before leaving home and then had a couple of slices of flapjack when I got to Cannock. I think one of my problems have been I have the wrong SIS drink for starters, secondly I don't often ride in a group so when I am in a group I tend to push that bit harder which then exasperates the problem, I am naturally a sweaty person so as quick as I am drink it it is running out of me. I had food to eat on the way round which I munched on at intervals. I did not feel totally exhausted when we finished, just the fact that I could not walk (although I did run to the loo when we finished ). I had no problem with cramp driving home although I did get a big twinge while sat on the sofa later in the evening.
At the moment my right calf is agony if I sit for too long and then try to stand up, almost like a torn muscle (this is what I normally get after one of these attacks).
I am going to look at the suggestions above and re work my drinking and eating strategy for when I am out in a group like Cannock. If I ride on my own I am normally OK