If you feel very strongly about this and all efforts to resolve the situation informally fail, the small claims procedure is very straightforward. Fill in some forms, pay a fee (which you can add to the claim) and you're on the way. No need for lawyers etc. If you get a judgement it will have a major impact on their ability to get credit, so issuing a claim might well be enough to encourage payment. However, it's obviously best to try and resolve the situation amicably.