Alex - this little "essay" might be of some help to you: http://www.stoneroof.org.uk/wdward.html
However, may I venture to suggest that your location in the UK is relevant to this subject. If you have a period property, particularly if it's listed, I assume you will want to use a roofing slate or tile that is appropriate to your area, in order to preserve the local character of your property. Such materials are invariably used close to their source (for example, Stonesfield slates in Oxfordshire), so perhaps your best source is likely to be not far away from you.
Thank you for your responses Penners and John and the links.
I had not been thinging so much 'tilestones' oe 'stone tiles' but had thought thickish natural slates of these varying colours with oranges tints etc. The location of my property is Guernsey and I am not aware of slate. I think hsitorically the local pantile dominated. The nearest location is probably Brittany and there are certainly fine examples there using them in places such as Dinan. I'm wanting to use them on a extention to my house which is not listed although 1600 and tradtional in appearance.
It seems to be relatively easy to find great varieties of slate floor tiles but much less readily available are natural slates.
They look like some types of Cornish slate, but unfortunately not like the current products available for slate roofing from suppliers in that county (carefully worded for a reason)