P1137 fuel trim bank one add too rich

Ok so I have a problem with a 1.8 t keeps on flagging up p1137 with no other codes

Started with basics put a new Bosch air mass sensor on it along with coolant temp sensor

Few weeks later code returns

So I replace the broadband lambda sensor

Still no joy long term fuel trim is still sitting at -21% which means its trying to counteract somthing rich

Has anyone had much bother with the pressure regulators failing ?

Car has done 100k full service history there's no air leaks or split hoses but that normally throws up a lean fault .car drives fine is completely standard

Looking for any advice on where to go next?

Thanks steven