Assuming that you're not considering going down the path of employing a couple of big guys with baseball bats, the only route open to you is as follows:
Step 1:
Send a formal, written demand to the debtor. Your letter (not email) must be dated, show your name and address and that of the debtor. It should be sent by Royal Mail's 'Signed For' service, even if you see the guy every day or if he lives next door to you, in case you need to prove to a court that he received it. Your letter should include the words 'Demand for Payment', indicating the amount owed and the reason for your demand. It should also include a statement indicating that if the money is not repaid within a specified period of time (I suggest 14 days) you will commence legal proceedings to recover your money.
Step 2:
Wait to see if you get your money within the specified time frame.
Step 3:
If you've not been paid back, commence legal proceedings against the debtor via the Small Claims procedure, using the official online claims process:
https://www.gov.uk/make-money-claim