Auto... seems like you're on the run around a bit now? Replacing sensors that don't usually fail and even mechanics scratching their heads? The one thing that would be useful to check is the ECM. A very expensive box of electronics and computer software, without which, the average fuel infected XX becomes a really good quality paper weight! The knack with this is not to buy one, but to try and borrow one. They're plug 'n' play units so if you have a mate or acquaintance with an infected 'bird, who will ride over to your house one afternoon, you can swap them over and you'll know if that's the fault or not in a few minutes (and frankly, you'll breath a huge sigh of relief either way!)
I am near you in Huddersfield but don't (and won't) own an FI model. I've stuck a post on Whippet (where everyone will look) but also, on the BIRD site there is a fella called Taffy, who apparently is not only a mine of info about the XX but has parts available as well. NOW... I'm not talking for Taffy here coz that's not on!! The ECM is the thick end of a grand new so simply 'lending one to a stranger' is a bloody big ask. Having said that though, I hear the guy's a world class gentleman with a habit of making things happen and problems disappear and I would certainly recommend you sign up on BIRD and drop him a line (if you're not already on BIRD)?
I hope this helps. What I'm saying is, that the fault you have might be the ECM but I'm not certain enough to recommend you stump out that kind o'f wedge without being sure
Eliminate the impossible. Whatever remains, however improbable, must be the part you forgot to service.