The first thing you need to do here is identify the legal person or entity with which you originally contracted. You should look at the quotation and any T&Cs provided by the builder in the first instance. Ideally, a company should be identified by its registration number, as a company can change its name without changing its legal identity.
If the documentation refers to a limited company, you can check whether that company still exists on the Companies House website. You might need to trace company name changes over time.
If the documentation does not refer to a limited company, then it may be that you contracted with the builder himself as an sole trader. In that case, a change of business name would not usually affect the validity of a contractual guarantee given by the builder.
It's not possible to give a definite answer to your question I'm afraid. That would require a full investigation of the facts. I suggest you speak to a local solicitor or CAB. You might also want to speak to your local Trading Standards office.