What to do when roof installed with no building consent

We used Laynes Roofing from your site and he has installed a flat roof with a rubber covering, on our roof, without building consent. Oxford City Council say we need a new roof as he has not put enough insulation underneath. We contacted him and he said it did not need building consent (the council says it does) and he will not put it right. Since then he will not respond to texts, calls or emails except on one occasion when he said he would come out to look at the roof. We waited in all day and he did not show and no response from him since. Do you have any advice? Also should you have him on your site as he installs roofs without building consent, that fail the regulations and then won’t respond to the client? I shouldn’t think his guarantee is worth much b