You have statutory rights that goods are of satisfactory quality, accurately described and fit for their purpose, if one or more of these conditions are not met it may be that your consumer rights have been breached, particularly if the builder did not point out to you the possible fault which you accepted.
If he is now not taking calls from you this is not good and it may be you will need to take court action. First write to the trader and ask him to contact you to resolve the problem within 14 days, and if he does not you will ask another trader to carry out the reasonable repair and that you intend to recover the cost from him. Check at this stage that he is solvent, there is no point taking action against someone without the means to pay you, and if he is a member of a trade body, if he is make a complaint to this body. If you are ignored have the reasonable work completed by another trader then write to him referring to your previous letter and enclose details of the cost to remedy his work, head your letter “Letter before Court action” and ask him to make payment to you within 30 days or you will take action in the county court with costs, if no payment is received contact your local county court and start action. A few words of warning, if your action in court is successful the court will not recover your money this will still be for you to do, perhaps with action by bailiffs, keep copies of all letters, and if possible obtain a specialist report on the work.