Sorry to hear of the issue.
The Consumer Rights Act 2015 applies in your situation - the work must be carried out with reasonable care and skill.
And the timescale agreed must be adhered to - the above applies in addition to contract law.
I am not what was agreed for the timescale though 19 weeks is a fairly long time and if the work is sub-standard, they should return to rectify anything wrong (under the consumer law, they are entitled to one repair attempt which if that fails entitles you to a partial refund or a full refund depending on the situation).
You could potentially terminate this (in writing), use someone else and sue this company for your loss.
Even if they get their act together and finish, you may still need to sue for any remedial work cost as they appear to be lacking with their customer care and communication with yourself.
A letter before action should be sent (which looks like the attached) to a) terminate and b) demand payment for your loss failing which you will commence county court proceedings.
You could do this yourself or you could use a law firm - in my experience you would make more progress using a law firm and it also puts in place a buffer between you and the other party - all communication would need to go to your lawyer and to you.
If you would like details of suitable consumer dispute law firms, please let me know.
Similarly if you would like further details of how to pursue court action, do let me know.