Nancy, thank you for your prompt reply...
A practical advice for your consideration...
How much would it cost you to put the matter right.. around £300?
You say the builder has become very threatening and abusive.
In light of this, I would stay clear of this builder and get the work done by some one else... You will find, it is a small price to pay in the long run, in relative terms.
Pursuing the builder may not achieve the desired goal but may be costly financially and also stressful. just write this money off to bitter experience.
You could report the matter to HMRC to enable them to carry out any enquiry into his tax affairs.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.