UK consumer law dictates that you have to give the dealer three attempts to fix the problem before rejecting the vehicle as not up to standard. I think i would double check this with trading standards and then write to the Dealer Principal / Sales Director, detailing the faults, dealer visit dates and your desire to reject the car. They will probably try to find a replacmenet vehicle, but this could leave you without a car or driving a faulty one in the interim. They will probably ask for the difference in price between your car now and the cost of a new one (even at 6 weeks old !). I'd stand firm for a striaght swap, again with advice from trading standards/citizens advice.
I successfully rejected a new car in 2006, but i never drove it out of the showroom (paintwork swirled on PDI) - they tried to revallet and then hide it with Zymol wax (the showroom smelt nice !!), but i wasn't having it. Not doubt a little easier to reject a car sat in the plate glass showrrom than one thats used, but i think you have a very valid case. If i can get a satisfactory result from Porsche i'm sure you can from VW.