Update. Oil sludge blocking strainer in the sump meant zero oil being pumped into the engine, causing the engine to stop & as a result the turbo is in bad shape with way too much play in it to be safe. Engine now turning but new turbo will cost around £900 inc vat. I don't want to risk a second hand one as don't know what to look out for & there are a few it could be (can't make out the numbers on existing one).
For my money I'd check/clean/buy a new MAP/Iat sensor. After having disconnected the MAF and run with it like that to see if it helps.
(I have had the same problem, diagnosed on here when I gave live data (Foxwell) while driving it. New sensor fixed it.)