We sold our car privately and the new owner took away with him the V5 document so he could tax the vehicle. He was then suppose to send it back to us so we could sign it and forward it to the DVLA.
A few weeks later I received a letter from the DVLA telling me that I was no longer the registered kepper of the car.
As I did not sign the V5 I can only assume that the new keeper forged my signature and sent it in himself.
Does anyone have any advice on how to deal with this matter? Will it be classed as fraud?

Crofty - it would help to note that you are asking about selling a car in the UK. Otherwise you'll get a bunch of Yanks like myself asking "what are V5 and DVLA?"

I don't understand why you let the buyer have the V5 - I believe when you sell a car in the UK you're supposed to send the V5 with the buyer's info and both your signatures to DVLA. I would think if he forged your name and sent it in then really no harm done, as you are now properly relieved of the liability of ownership of the car.