If it was a limited company, then my understanding is that your personal liability for that company's performance is limited, usually to £10. The point is that the company ceased trading and is not you as an individual, so you need to be very careful not to portray yourself as still connected to the company (in spite of the fact that you were the owner back then). So do get proper advice on what not to say or write.
EDIT Just re-read the opening post. When the OP said 'the company ceased trading', I read that to mean it was a limited company. Not sure, on re-reading, if that was the case at all!
Back to liability in general, you are responsible for carrying out the work with due skill and care, and to hide behind the wording of a warranty will not help in court. The warranty is in addition to statutory rights and those rights cannot be deleted by a contract. In practice, firms often do try to hide behind warranty terms and fob people off, but they cannot get away with this if it goes to court.
In practical terms, though, I'd like to know how this customer intends to prove that the failure of the joint is somehow your fault after all this time. I think they will try to push you for an out-of-court settlement because I expect if you dig your heels in and refuse to give one, then they will probably lose the case and leave with nothing.