Write to them setting out the facts - dates, amounts, etc., as much info. as possible. Politely advise them how much you are owed, and offering them seven days from the date of receipt, to make payment, or you will pursue the debt through legal action. Send your letter Recorded Delivery, keep a copy, and your receipt.
If you get no joy, ask a solicitor to fire a warning letter at them - should be done for a nominal fee. If that fails, contact the Small Claims Court and see if you can take action that way.