"Fred Bloggs,
As a result of your actions, causing damage to my car, on
(date), I have suffered financial losses as follows:
(List all the money you've paid out)
I therefore DEMAND PAYMENT, from you, of the sum of �
XXX.XX.
TAKE NOTICE that if full payment of the said sum is not received by me within 28 DAYS, I will pursue this matter through court action"
Ensure that your letter clearly shows your address, and is dated. Then send it by recorded delivery (even if the guy lives just a few yards away from you) and retain the proof of posting.
When (as will almost certainly happen) he doesn't pay up, follow the procedures here:
http://www.hmcourts-service.gov.uk/infoabout/c laims/index.htm
Chris