Hi - i have recently had a re-roof on a mid victorian terrace. They used existing slate and new slates. I have paid half the money but am reluctant to pay the balance as in the loft there is rubble in between the slates and felt. I can see chinks of daylight. The slates dont sit flat on the roof. Nails are sticking out of the felt. I dont know what to do. Can this be salvaged. He said he would remove the rubble-from the inside? Can i have advice please.