To make a claim for unpaid invoices in the UK, fill out Form N1. Below are some sources from which you can download Form N1. Use the guide here to help you fill out the form.
For claims of £100,000 or less use this website.
Money Claims Online This is a HM Courts & Tribunals Service, an internet-based service for claimants and defendants. Please note that the service is still in beta mode and you’ll be required to fill out a few questions. You’ll also need to create an account with money claims to start using this service.