There's been that much going on I forgot that one, sorry Nigel. So we have two ideas to eliminate slippage, so much the better. I've been thinking about Steves rotating pivot log and how to secure it to the stone. So here goes, use a fairly substantial pivet log, place a similar log on top of the stone directly above the first, join the two together with logs each side of the stone using motise and tenon joints with an extended tenon. (like the legs of a refectory table) When the wedges are driven home it will grip the stone like a vice. If that's still not enough do the same again further down the stone (about three foot) and join the two rings together with cross bracing. That should hold forty ton for the time period required.