The writer above is correct, they cannot get a CCJ against you without Judgment so I wouldn't worry about this as your first concern.
Without more information, it is difficult to give you accurate advice but if you have/had a contract with them, check the terms of that agreement (I know you said you didn't have specifics for each job but did you have any form of written agreement?). It may make some reference in the contract about who the work belongs to before and after payment. If you are employed by a company, the work that you do in their time usually belongs to them so in effect, you may be stealing from them! The contract may also say something in the contract about the payment terms. Check who is breaching what term basically. If the money you are owed is for the work you are withholding, perhaps they have a point of withholding the money.
If the contract is fairly clear, then you need to decide on your tactics. I would have thought the best thing is to find out why they won't pay you. What are they afraid of or can they simply not pay? The second option is to take the case to them. Put in a demand for the money and set out when the days were and the hours (if important). Give them a deadline for payment. Without seeing the contract, I would advise not putting anything in the letter about withholding the work. If the deadline passes, send them a letter saying you are now taking the matter to Court and issue a small claim. They may well counterclaim though so try to avoid getting to this stage.
Hope some of this is helpful.
Good luck