You should just track him down and phone him, or visit him if you live close to him. That's precisely what I'd do, go speak to him man-to-man about it.

It's actually very easy to track people down on the Internet with a little bit of research, if you need any help, just give me a tap on the shoulder.

If you feel it's worth it (More the principle than the amount), take him to a small claims court!

http://www.bbc.co.uk/crime/law/smallclaimscourt.shtml