You should get yourself a scan tool, preferably VCDS. It's an invaluable tool for these cars.
Need to see what the MAF reading at idle is and whether or not plugging the N80 input on the manifold changes that value. The reason for that test is cost. A new MAF is £140ish, and an N80 valve is considerably cheaper. It just avoids firing the parts cannon at the car and hoping for the best. There is usually a code associated with a faulty N80, usually "Evap system - incorrect flow".