It was good speaking with you just now. Consider sending the builder a letter before claim where you will set out all relevant facts and require certain action i.e. refund of your money as they failed to complete the works and breached the contract. Have a look at a free template here: Letter before small claims court claim - Which?
Generally, the letter will contain the following:
your name and address
the reason for your claim
a clear explanation of the facts
what you require the builder to do to put things right
how much you're claiming
a list of any documents you are referring to such as the underlying contract etc.
an invitation to use mediation to resolve this matter amicably
a date by when you want the builder to give you a full response i.e.14-21 days
a reminder that you will issue a claim in a small claims court if the builder doesn’t reply to / comply with the letter
If the letter does not work issue a claim in a small claim court. You can do it here: Make a court claim for money - GOV.UK (www.gov.uk)