Sod that. Raise a MCOL (artist formerly known as Small Claims Court) claim. Just be careful to follow the pre-action protocol, which is basically to make sure you write a Letter Before Action following the guidance in Appendix A of the Practice Direction for pre-action protocol (the MCOL website points you in the right direction). Straightforwardly state breach of contract as the relevant law. Make sure you show all the relevant elements of contract are present (your offer of X goods and services to be supplied in exchange for £Y, their unconditional acceptance of that offer, intention to form legally binding relations, and consideration [consideration here means you're going to supply goods and services which they're going to supply payment for - basically that the contract includes things of value are passing both ways]). State that you will be adding your court costs in with the amount claimed. Say that you are open to a means of resolving the dispute other than court however you will not budge on the amount owed (as long as what you are owed is indisputable). Many will pay when they realise from the Letter Before Action they're about to be taken to court. I know this all may sound a bit intimidating but once you've done it once you then have a template for every time you have to do it again ever.