BLOB,
I recently completed the roof of my self build Dormer with Cupa Heavy 3 Natural Slates from Brooks ( I have no connection to Brooks). They are a spanish slate and cost 1:38 +VAt per slate. I'm very happy with them and they look great. They do need to be sorted as there is a big variance but any good roofer should have no problem with them. There are also impurities (pyrite) as with other natural slates. For this price though it is to be expected unless you are willing to pay 3 times the price for Blue Bangors, etc...