Sorry to hear of your issue, but well done for the diagnostic work done so far. Could this be an injector seal leaking I wonder? Or bad injector in general?
Just to add if you're needing someone to fit it, then it's worth looking at autologics or Audi retrofits as members have used them and been pleased with the results.
There's a lot of documented case of motorway chargers not working.
If you bought it on finance it might be worth contacting the finance company about your issue.