When all other means are exhausted, you can begin small claims proceedings. Contact your nearest county court (under Courts in the phone book) for leaflets and form N1, or download them from the online at www.courtservice.gov.uk.
The form itself is simple: you give your address, and that of the defendant. You must get the address right, but remember that the defendant can ask for the case to be heard at his nearest court, so it may make sense to give the address of your local branch where the problem occurred rather than the head office.
You add brief details of the claim, and the value. Enclose details of the claim, including invoices and your attempts to recover the money without resorting to law. Add any solicitor's costs, and add the court fee as detailed in the leaflets which come with the form - £30 up to £300 claimed, £80 for up to £1,000 and £120 for up to £5,000. Add the amount claimed, and make out a cheque for the total to H M Paymaster General. Then send in three copies of the form, and you will be given a claim number as the court begins proceedings.
Your client has 14 days to respond. They can do several things, including ask for another 28 days to prepare a defence. They can ignore it completely, in which case you can ask for judgement in default since they have not replied, and payment forthwith or in instalments. They can admit the claim, and offer full payment plus your court costs. Or they can file a defence or a counter-claim.
If it's the latter, you may join the small proportion of cases which actually reach court each year: the vast majority are settled soon after your claim drops through the client's letterbox. You then get a copy of the defence, and you and the client are sent an allocation questionnaire to help the judge decide how to process the case - long or complicated claims can be allocated to the fast or multi-track, whereas smaller scale claims go through the small claims track.