Amazingly it arrived today not long after I'd posted here - it is indeed just a bit of slate in the shape of an oilstone, rather flatter than the bits I found on Llanfarian beach a couple of weeks ago. I have used it, initially with water, then as it got gunked up with the remains of the oil I use in a futile attempt to keep rust at bay, with oil. I must say I was pleased, I don't have a huge ammount of experience, so feel free to disregard my comments, but I can imagine being quite happy to go from a Norton fine to this to a strop. The only slight problem is slate is easier to scratch so requires a little more care, the upside of that is hopefully it'll improve my technique on oilstones and stop me hashing up my chisel corners. It's also cheap at £7+p&p and, although nothing to do with sharpening, it's nice to know that my money is staying in the UK with people making something.
One final thing, Inigo Jones' site specifically say it is to be used with water, which makes it a waterstone - a Welsh waterstone, apart from the rather pleasing alliteration, is this any more or less acceptable than Japanese waterstones, given that slate has probably been used in this way since time immemorial.....
I'm retreating to my bunker for now but here's a link to a review by someone who seems to know what they're talking about.
http://www.britishblades.com/forums/showthread.php?130020-Inigo-Jones-Welsh-Slate-Wetstone
Cheerio,
Carl