You should think very carefully about whether or not to make a claim against your client. Although lodging a small claim against them may mean you are able to claim back the monies you are owed in the long term, you should think about whether this might damage your relationship with your client. If you get your money back but lose a client in the process, this may not be the best course of action to take.
You should first check how likely your client is to pay you back — you can check their company profile in The Gazette or Companies House to see what their finances look like, and whether they are solvent.
You may then want to get legal advice before going any further. Remember that you can send a solicitors’ letter and serve notice on your client; this may prompt them into paying before you ever get to court. You should make sure you have written evidence of trying to resolve the issue with your client before commencing any kind of legal action. If you haven’t done this step first, it could count against you if the case ever goes to court.