Me letter had a good effect.
A few days after I sent it ( to VW HQ ) I got a call from the quality control officer at the garage I took my car to (Rye Mill in High Wycombe). She apologised etc, and offered to give me 50% off my next service. However, I never wanted them to touch my car again again, so after some haggling, she agreed to give me the ca$h equivalent ( ?150 ). A cheque arrived in the post shortly after

She also moved the body-shop appointment forward to this week.
Just got my car back today.

They've resprayed the whole bonnet, and bumper. Looks good as new now, altough they've left some dirty marks on the seats (hopefully these'll just wash out).
Was deffo worth complaining!