Spoke to the quarry manager, he says ten tons is no problem, and even implied they might be able to produce one of the right shape to order from a particular part of the quarry. However...
Before any decision he'd need a Description of the method, a Risk Assessment and details of our insurance cover. He thought this would be a standard reply throughout the quarry industry.
I guess we'll need that stuff for the final phase. Do we want to go down that route for the earlier ones?
If not, then we're left with looking for a friendly farmer with a stone - much simpler, but can we find a ten tonner?

I did find one elsewhere on the hill, 8 ft x 5 ft x 4 ft, round bottom, pointed top, perfect for raising, in a rock-strewn location very reminiscent of the source at Avebury. Remote and not overlooked so paperwork wouldn't be a problem. It's such a jumble a bit of moving would hardly be objectionable. Not much room to row though.