I need some technical advice from a roofing expert

I had a flat roof replacement carried out. This wasn't a repair. The roof was sound and had survived months of extremely bad weather without any leakage. A few days after the work was carried out, the water from a very hard downfall of rain caused a leak through the ceiling. It has completely ruined the cosmetic look of what was a brand new kitchen.and is going to cost £260.00 to repair. When one of the employees came to inspect the roof I was told that someone had not sealed the roof properly. The owner has told me, even though he has entrusted this employee to inspect the damage that it was not that but it was the side flashing that caused the leak which he supposedly fixed afterwards. So, he is not admitting liablility. Is this possible especially as the leak had not occurred with really bad weather prior to the work being carried. As i say this was not a repair to the roof it was a replacement I had had no leaks before this time.. By the way unbeknown to me this person is not a member of any association. He was in fact recommended to me by the builder who installed my kitchen who had known him for years and had used him for work himself. That and glowing reviews elsewhere too was enough for me..