Ian,
I love slate and have used it on several projects I have done. There are a couple of things you need to note about slate if you don't know already. Slate, or any natural stone, will require a sealer after it is installed. The sealer will change the color so you may want to buy a couple of pieces and seal them to see if you like the look. Slate is also sold two different ways. There is gauged slate, which is consistent in thickness, and ungauged which will vary not only from tile to tile but also with in a given tile. Ungauged slate is much harder to lay without leaving edges that will stub a toe. Also because of the irregularities of slate it is harder to keep clean. Mopping tends to leave residue in all the little variations. Keep that in mind while grouting or removing excess thinset mortar. Make sure you keep up with the cleaning before the mortar starts to set.
Don't get me wrong, I think slate is a beautiful look but it is harder to install than a regular ceramic tile, in my opinion.
MarkV