Then you need to pursue your debt through the civil courts. You must send them at least 2 letters first, then a 3rd with your legal intention of suing them.

Bear in mind, you can recover your court costs, but not solicitors fees (otherwise everyone would hire the best and most expensive solicitors!) so you need to decide if the debt is worth it.

Or you can let a debt recovery company buy the debt from you at a reduced rate, and they chase it it for you?