I have a Large Warmbac Fleece under-suit for sale, bought it off ebay a while ago, but it's a bit on the snug side for me. It's almost brand new.
One piece stretch undersuit, warm and lightweight, tailored for maximum freedom. 2-way zip, diamond crotch, pocket, neoprene cuffs, double thickness at knees.
I paid £30 for it (cost £50+ new) so offers around that would be good. Need to sell it soon so if no offers it's going back on ebay.