JP - that is a brilliant explanation and I think that is pretty much what I was trying to do - but after fiddly about for ages I did end up chucking the offending trousers in the bin. I had got the tummy looking reasonably ok but then the legs and crotch area were really baggy and I couldn`t face messing about with them any more.
Maybe I`ll have another bash one day - but not in the near future!
(I`m sure your instructions will be really helpful for others though)