is this right....synthetic slates used on natural slate roof

[Chappy]

I cut through copper nails with my ripper only last week when I replaced 2 broken slates.

As for synthetic slates, a builder I worked for in the eighties built himself a huuuuge stone house - I did all the labouring on it - and we did the whole roof in synthetic slates. He still lives there now 30 years on and is still impressed with them.
They look great now they are nicely weathered too.