Builder breached contract now threatening to take me to court.

I have had a contractor do substandard work on my conservatory, he has breached his contract with the standard of work and materials used. He refuses to return to put everything right just some of it. He never gave me a contract but the agreement was payment on completion . The job started twenty months ago and should have taken a few days. The job is far from finished as much of it needs refurbishment and remedial work to correct his faults. By not returning to put everything right he will be leaving me in a position of having to pay another contractor to put his work right. He is demanding full payment now row works completed and if I don't pay he is threatening to take me through the courts.

I have offered to pay him half the amount now pending completion with the provision that he rectifies all the faults and ensures this job is completed to a satisfactory standard ,this time using skilled tradesmen and the appropriate materials. He has rejected this and is demanding full payment and is now threatening to take me to court. He has not fulfilled his contract with me by not providing me with work done using reasonable skill and care and appropriate materials and a reasonable time scale. It's been twenty months now. He has also performed illegal gas work leaving us with a carbon monoxide leak, he has charged us for work that wasn't possible to do Any help or advise would be greatly appreciated.

Builder breached contract now threatening to take me to court.