Thanks everyone for your help. I've contacted my local NAD dealer. They didn't want to release the specs for the driver, but gave me a quote for a replacement, which was £60 ($115). I would have liked to buy two new drivers like Philip said, but I don't think that the quality gained would be...
thanks for your help, I spent ages looking on google for any info, but couldnt find the NAD website. After looking on the NAD website I think that it must have been a custom job, as the speakers are only about 5 years old. I've got the phone number for the local dealer, I'll phone them to ask...
Last week my fathers boss, who produces his own music gave me his old speakers which he was going to throw out. He said he paid £200-£300 each for the speakers, but after blowing one of the drivers he wanted to upgrade to a better pair. The voice coil on the blown driver needs to be replaced...