Thanks for the warm welcome guys. The car's not new, its nearly 18 months old and has done nearly 15k miles. Up to now it's been faultless and goes like a train. I had a MK4 V5 before this which I had remapped to almost the same bhp as my GTI. I do miss the 5 cylinder engine note but the GTI is fantastic. Handling and response is a world away from the V5, and it does nearly 40mpg!
I'll see how it goes over the next week or so but it will be in the dealers for a service soon so I'll get them to check it out, unless it happens again in which case it will be straight in.
Incidentally, mine has an annoying rattle coming from the steering column. It isn't mechanical and has been into the garage to be investigated once. They did cure it briefly but it's back! Anyone else come across this?