Well guess that still leaves fuel pump and FPR, only thing is I don't know how you'd check that either of these was working properly without swapping it for another one. Have a search on here or post, don't think FPRs are very much to buy but fuel pumps are about £80. You could check the vacuum hose that goes to the FPR, think it runs under the manifold.
Cupra Ross had a thread on a similar issue to this, think I posted it in this one already. Turned out his
problem was the fuel pump. I'll try and find that thread and have another read over it.
Might be worth finding other folks that have had a Forge remap on their car and see if any of them have had similar issues?