Cheers Leeroy......thats really helpful.
It seems to be cured since I slung the aftermarket chip.
Its still pulling fine, not kicking out any dirty crap, doing 50-60MPG so seems to be all good in the hood.
I did discover that I didn't have to knock it into neutral to get it to respond to the accelerator though...........the revs just had to drop really low then it would catch again.
As I said though my feeling is it was the chip. Changing the fuel filter tomorrow though just to be on the safe side (its was due a good service anyways).