Stuart - I do a lot of skiing. My Gore tex is really fine (as in thickness) yet keeps howling snow storms out at -20oC! The real benefit is that it also breathes as well, so you dont get too hot in it either.
As Barney says, have a look at Army surplus. I got some really light weight gore tex army over trousers for ?15, and in winter I just wear them over some tracky bottoms. They are brilliant. Equally, the Sundridge is very good as well.