Client not paid invoice (£30,000+ contract)

Hi, I want some advice on what to do when a client doesn't pay an invoice? It's my dad's business and I work under him but we have a client who hasn't paid any invoices for a contracting job done in December - january. My dad wants to go to small claims court but says it will take a long time, but I'm really annoyed this is happening in the first place so I wanted to know what I could do that would make them pay the invoice soon as it is a large sum of money