Dispute with roofers

Maintenance

Four years ago I had a flat roof at my property replaced with a fibre glass roof by one roofer. Last year I had the rest of the roof, pitched slate, replaced by a second roofer. Twice during recent torrential rain the roof has leaked near a joint of the the two rooves. There had been no leaks until recently, but the recent rain has been unprecedented and the second roofer claims the surface of the flat roof has degraded over time. Both rooves are covered under guarantee but both roofers are blaming the other's work. I am caught between them with apparently no cover. What options do I have?