its a Hardrow slate, some times nailed but can be pegged too.
should e easy enough to find.
no it is not water proof, but your felt will help some what.
any problem find one then there is a roofing reclaim yard i know has thousands. but depends on your location
HI again, thanks for your help. Could I just ask the name and location of the yard that has thousands of these Hardrow slates? We are going to see mother in law at New Milton soon.
Many thanks
PS roofer fixed them back in place OK so we're now watertight!