Hhmmm...thinking aloud here. I'd seen the diamond trueing thingy on a stick on Axminster. Problem as I see it is not only is stone 'domed' slightly, the wear is uneven around the periphery - theres a real low spot. If I flatten it on glass, I'll get it flat but maybe not even thickness so it'll still undulate at the periphery. I could use the devil stone but same problem perhaps. But if I mounted the trueing 'device' on the sliding tool holder, I could get it flat and level guaranteed. Maybe adapt the diamond stick thingy i.e bend the handle so I can clamp it to the toolholder so it'll come down flat onto the stone (making the angle adjuster a height adjuster), or maybe I could true it back just by sharpening a planer blade (shame I haven't got a spare one that can be sacrificed).
Thanks all for the suggestions - any further input more than welcome.
Ike