Thanks for your replies everyone.
I had a look at the sureflap site. it seems as if they do one that fits the need.
It needs to be battery powered, so that I don't have to run mains power to the door. That would present a problem. it could be done, but I would rather not do it.
As for size, he is still not fully grown, you should see the size of his dad. he's enormous! Quite the largest cat I have ever seen. But It never ceases to surprise me the places a cat can squeeze through.
Its' not the poo thats' the real problem, although thats bad enough; its' that he is very 'gassy'. Incredibly so. I think its a change of diet and I am hoping it clears naturally. As he has come through a number of in depth vet check ups very recently, I do not think it is anything more serious than that. But my wife reports that it is seriously offensive to the nose, even the other cats complain and the fish dive to the bottom of their tank to get away from it. I'm OK, as I am several thousand miles away, safely in Afghanistan.
All of this aside, he is a very handsome cat and quite sociable with other animals, including our huge rabbit (6Kgs French Lop).