Update time again guys.
Turns out Vauxhall had supplied a none genuine Ebay MAF, but charged VX prices (£120) and sold it in a GM box with hologram on it. Went to the local auto electrician who played about with it. Eventually he took the MAF off and turned it over. Missing a serial number and Siemens wasn't printed on the bottom. He ordered a new genuine MAF from his supplier, showed me the bottom where I could clearly see a serial number and Siemens in the plastic, put it in first time and all was great straight away.
I went straight back to the dealer where I got it, with the old original sensor and the one they supplied and got the parts guy to play spot the difference. This MAF was brought in from another dealer and he'd had problems with them previously. I'm going back tomorrow with my reciept and original packaging for a full refund.
Selling aftermarket MAF's at GM prices in the GM boxes when I could pick up the same thing off Ebay for 30 quid is really not on.