Bit of a blow. Looks like the roof will have to be completely re-tiled as the tiles are faulty. We thought they were just breaking off where thieves had ripped the lead off the roof before we bought it. But the problem is more widespread - a lot of them have got hairline cracks and are randomly breaking. It's a lot of roof to re-tile.
Met the neighbours who said the tiles had come from China and were very cheap. Maybe this was towards the end when the previous builder was running out of money.