Having read through the previous posts, normally a deposit is just that, sometimes you get it back sometimes you dont, but it is normally stated either way ie: a non refundable deposit.
If it was me i would bite the bullet return the £300 and be done with it, last thing i would want is waking up one morning and finding a dent or scratches over my car. He may/maynot know your address but if you google your name its easy to find, esp if you use facebook etc.....