Hi Geoff, Simon and Paul 23 are almost right, you must issue the person or institution a letter before action, and as stated it must include the exact amount you are owed, and a fixed date for it to be paid.
However do NOT send it recorded delivery, as if he refuses to sign for it, he has proof that he did not receive it.
You just have to get proof of posting from the post office, and start the deadline from 2 days after you post it. That is a legal proof of receipt then.
If after your deadline he has not paid in full, then go to the moneyclaim website, the judge is more inclined to favour you if you follow all the rules, that said once he receives a court paper on his mat he may just pay quickly.