I think you might be right!

If I'm right the halfcut stone is lyng in front of the Drakestone, it's just visible in the bottom left of this: http://www.themodernantiquarian.com/post/43310 photo. It's about the right size to have come from the larger basin, and has the same strange patterns running through it. The same fossils (or whatever they are) would probably have been what made it split in half, it doesn't look like good millstone material really. There were definitely folk with chisels up there in the late19thC as the graffiti on the reverse of the stone attests.

That might explain why the references to the 'Druidical' basin have been discounted. Anyone aware of them would probably first notice that millstoney thing and think "Ah, that's no Druidical rock basin..."

Cheers for the astute observation Creyr, it's most appreciated. It's a wonderful landscape is Coquetdale, did you ever get up to anyof those hillforts around Alwinton?