You could take him to court using the Small Claims' Procedure - it's cheap and easy.
However, you will say he owes the money, he will say he doesn't. Even if the court finds in your favour, it is hard to actually get the money - you may have to pay again for bailiffs, for example.
If you think he's the sort to be very worried at getting a court summons he may pay up before it ever gets to court - in which case it may be worth sending him a summons. You can do it online. You would need to write to him, stating he owes you x amount and if you don't receive the money within 28 days you will take him to court. You would send the letter by recorded delivery.
Then issue the summons 30 days after you posted the letter.