In winter I use a 7.5mm cheap one-piece dive suit from a k-mart type store here in NZ. It is brilliant. Easy to get on and off and really warm. It's so warm I have to pull out the collar to take big gulps of cold water every so often to prevent overheating. Cost about NZ$160 (90 pounds).
One problem, i didn't tie it on my trailer properly and it disappeared. They don't sell them anymore either so i am off to slit my wrists... goodbye cruel world!