Hi Lisa,
same problem as you late last year.
It really is very simple:
Legally, he should pay you, then take you to court if he has a complaint. But he hasn't done that, so you are one step ahead already.
The first thing to do, is to get all your paperwork together. Look at it and make sure you have some form of agreement with him in writing. To be sure that he is in the wrong.
Go to:
http://www.moneyclaim.gov.ukFill out the relevant form for your situation and pay your £30 or so. (It goes up, the more you are claiming - but yours should be about £30)
On the very simple form, explain the situation. That you have an agreement, you have done work and that he is refusing to pay. Give all the information, but not in great amounts of detail - keep it simple and to the point. Keep a note of all reference numbers and phone number for the court.
Then submit it! It's as simple as that. Within a few days he will receive the claim from the court. He has 28 days to contest or pay up.
Chances are you will get a cheque (without your costs) just before the deadline.
Just call the court and tell them that you have recieved 'part payment' and are returning the cheque, asking him for your court costs. Then let him know what you have done (don't forget to return the cheque - copy it first, if you can) The court may give him a bit more time and I bet the next cheque follows soon after.
As soon as it clears, withdraw the claim by phoning the court.
I wouldn't bother with interest on anything less than £5,000. It is very complicated to work out, it is accumulative and the amount you would get back - even if payment has been overdue for six months - would be pennies!
Hope this helps and good luck.