A court won't entertain any claim by you unless you can show that the debtor has failed to respond to a formal written demand for the money owed.
So your first step is to write a letter. It must be dated and clearly show your name and address, and that of the debtor. It must also clearly state that it is a demand for payment, stating the amount owed and giving the reason why. The letter should also state that, if payment in full is not received within a specified period of time (I suggest 21 days), you will commence legal proceedings. Send the letter by recorded delivery, keeping a copy and using Royal Mail's online tracking system to confirm receipt.
If you then don't get your money, start here:
https://www.moneyclai...v.uk/web/mcol/welcome
Chris