OK Guys I've nearly got my head right so.

The side force only comes into play when the stone starts to slip down the tower, but the rotational force that has built up in the stone means that this slipage only ocurrers for a short period so that as the rotatioal force takes the top half of the stone away from contact with the tower we're OK again. Steves long wooden lath would not have been heavy enough to build up much rotational force so it pushed the tower over.

In the experiment I did with the 4 ton block the pallets only moved a matter of inches. If we buttress the tower to hold the force for a second or so the little bit of slippage we get will probably take some of the rotational force out of the stone and make it work better.

The good news I was on about earlier, I've had a word with the manager of the museum and he has promised to get in touch with the manager of the quarry which is on the same site with a view to them supplying some land and a ten ton stone. He was pretty confident of the result.