Cheers RRB,
I've had a good look around and there appears no obvious fault.
I have a snorkel fitted and I'm now guessing that when I'm on a longish run the flexi pipe could be getting warm enough to collapse,
I have a wire reinforced pipe but it is a bit on the thin side and although the wire should stop it from collapsing, it could collapse in a "domino" type of fashion and close down the air feed to the engine,
This would give me black smoke and a loss of power,
it would also appear to be running rich.
I'm in my mates garage on Saturday and will give the turbo a good test and try and find if there is any other problems with all his professinal testing equipement.
I'm going to change the air feed pipe into mostly solid pipe and see if that changes anything.
Cheers for now,