Hmmm, sounds a bit dodge, and probably cost 30 quid?
People can and do fit aftermarket pumps into the stock housing, that's not an issue. The problem is you get fuel starvation (aka surging) on corners when the tank is about 1/4 full and lower.
Try googling continental, VDO and "pierburg in-tank pump MK5 GTI"
If you see just the metal pump listed, happy days. If not, assume nothing exists. I think the factory part is unique in some way.