Just to try to bring together the essential elements of the answers above:
1. If the guy who was driving your car was not responsible for the accident, you can sue the driver who caused the accident (who will refer the matter to his insurer, who'll pay you). That applies even if there was no valid insurance covering the person you lent the car to. (However, that doesn't prevent you both being prosecuted if no insurance was in place. See '4' below).
The following applies if the guy driving your car was responsible for the accident:
2. If the other driver is a 'named driver' on your comprehensive insurance (or in the very unlikely case of you having a policy which allows anyone to drive your car, while retaining full cover), you can successfully claim against your insurer.
3. If the other driver has his own insurance policy, for a different vehicle, (and that insurance covers him to drive any other vehicle), he'll have been driving legally but you can't claim a penny from his insurer (because his 'any vehicle' cover will be '3rd party only'). However, you can sue the guy who was driving your car, to get him to pay you from his own pocket.
4. If there was no valid insurance in place, you can still sue the guy driving your car, forcing him to pay out of his own pocket. However, both of you have committed a criminal offence. You both face 6 to 8 points on your licence and a fine of up to �5000.
Chris