UPDATE-

Got back on this one again at last (hectic few days)
I am leaning towards the internal air leak theory with this as if when the vehicle is running smooth you remove the oil filler cap it induces the exact same symptoms, even down to clearing up when the air mass plug is removed (with the cap still off)

So clearly an air calculation issue and not timing. Any pointers on where this is likely to be?

Will update if anything is found.

Thanks again