Barry, I've done alot of searching for natural slates. Slates direct have varly competitive prices, beware of brazilian natural slates though. They've a repetition for being brittle, and some require additional clipping at the bottom. There is also alot of wastage sometimes with cheaper natural slates.
I'm building myself at the minute and am going to use a reconstituted natural slate by Capco (cambrian slate);60% natural slate and 40% resin. It looks good in the flesh, and unlike natural slate there is no grading required. It has a 60 year guarantee, and is meant to weather just like a natural slate.
Best of luck with the natural slate if you go with it.