You've not said what phone it is, but all nokia phones come under a warrany which allows you to take to any nokia repair center (usually carphone warehouse) who will fix it FOC. All they check is the serial number of the phone to find out the warranty information.

I bought a nokia 6230 off ebay back when they had just come out and were very hard to get hold of. It had a fault with it and the software had to be realoded. I took it to nokia center who checked the serial number, verifiied it was underwarranty and repaired it FOC. It didnt even matter that I'd bought offline off ebay.