Right - managed to get may hands on a second hand MAF that I've seen working properly on another GTI.
I'm gonna replace the air box / MAF / and all the pipes around the ISV and see what's what after. Looks like a nice game of elimination really, but seeing as the MAF, air box etc cost me ?20 I'm not too fussed.
Hopefully this will cure it and I can sleep better if I spend ?99 upwards on a new MAF, I'd go spare if I forked out for that and it didn't solve the problem !!
I'll keep you all posted ....