Matt, a couple of things, does it sound quite rough as well? you have covered most of the bases, I had a problem about 4 years ago that sounds similar and it turned out to be the exhaust gas control valve not the little switchie valve but the gert big one between the turbo exhausts and the exhaust, they are known for jamming, mine was stuck open, which gives you similar symptoms. with the engine off, see if you can reach down to grab the actuator arm and see if you can move it, this is not easy though, you may have to put some mole grips on it so you can give it some, even normal operation it takes some moving.
Just a suggestion if all else has failed, in the end I fitted another 2nd hand one, never happened again.