Massive thanks to BBB for the lend of his skills, his driveway, and hours of his time today helping me install (and doing most of the work) my new turbo inlet.
As suspected the factory inlet was my problem. This pic should explain why...

From the marks on the pipe it looks like it was originally installed correctly, but at some stage it's been taken off and reinstalled incorrectly. I don't remember ever taking it off so take from that what you will.