So I finally fixed my turbo limp home issue, but not in the way I expected! I was completely thrown off the scent by the diagnostic errors so thought this might be useful to someone. The car was showing overboost and N75 open/short to ground intermittent errors. First thing I did was vacuum test the lines and check the N75 terminals - all good. So I thought, it's probably a failed N75 and a clogged turbo. New N75 valve fitted and the intermittent fault went away, just the overboost was left. So before totally stripping the turbo I tried some oven cleaner to see if I could avoid the teardown. This significantly improved things, no overboost fault, but.... The N75 fault was back! So I resigned myself to stripping the dash as I thought it must be a wiring issue from the N75 to the ECU. A clogged turbo couldn't cause an N75 open/short fault right? Wrong! A long run later (8hrs) purely by luck has totally cured the turbo issues. 1 month later and still no problems. So take note, and don't believe everything the diagnostic companies tell you about fault codes and their causes. Hope this helps someone!