Just to update you all and thank you for your loving replies.
All seemed to go well for Sophie after her neutering.
Then I got up one morning to find blood in her stool. She went to a new vet who 'expressed' her anal glands and said that she may have this problem on and off for life; having to go to the vet every three months or so to have the anal glands emptied.
Anyway, the next day she stopped eating wet food of any kind and has hardly eaten anything today except a handful of nuts and a morsel of catfood. She is drinking ok and seems to have energy.
She has not been to the toilet for about 24 hours.
I took her to the new vet today -a fifty mile trip- and the new vet examined her, took her temperature and said she could find nothing wrong, she has not got an infection or anything stuck in her gut.
And if I said my cat has not done any poo for 24 hours or barely eaten for 48 you would all cry "Go to the vet!" But the vet does not know either.
So yet again I have to go to bed worrying with mounting vet debts. I've been in cat hell for awhile, hope things improve.