Get a fixed quote from another builder for remedying the problem.
Then write to the original builder stating that if they have not fixed the problem, or paid you the sum quoted by the other builder in order to fix the problem, within a specified period (I suggest 28 days) you will commence legal action to recover the relevant amount.
Your letter must clearly show your name and address, as well as that of the builder. It must also be dated. Send your letter by recorded delivery (retaining the proof of posting), keeping a copy to show to a court if necessary.
If you don't get your money (and the builder has not rectified the problem) commence your claim using the online system provided by Her Majesty's Courts Service:
https://www.moneyclaim.gov.uk/web/mcol/welcome
If you'd prefer to place your claim before a court by the conventional route (and/or to find more information about making claims), see here:
http://www.hmcourts-s...bout/claims/index.htm
Chris