You've got me worried now! I sold a C15 last year and have just received the tax reminder. However I called the DVLA and they just asked me to return the form with a letter saying that I had sold the bike and that would be the end of the matter for me. I didn't have the buyer's details either, but the DVLA seemed confident that when the new buyer went to tax it, it would be discovered who the new owner was. I guess, if he chooses not to tax it, then the bike will remain in limbo. I hope they don't come chasing me!
Nigel.