When houses "suddenly" move it's usually due to tree roots or water leaks or mining activity. I presume none of those applies here but the movement is relatively "sudden".
The only other causation I can recall is a whole line of terraced houses in West Bromwich that suddenly began to settle along their front walls. It was eventually diagnosed as dodgy footings,obviously, but finally precipitated by traffic vibration. I wonder if there could have been heavy lorries or vibration such as driving in piles round there in the past few months/years?
It may be nothing like that Nigel. Look at the angle the capstone lays at and the thrust it places on the closure stone pushing it forward. It is over 21" off the perpendiculat at present and moving all the time. That was what probably set it all off after the stub tennon tore off. BTW, I was reading your earlier post again where you mention Stonehenge and I don't know if I read it correctly but were you assuming I thought the stub tenon went into the 'hole'? Well I didn't, it would of had a separate stubby mortice which would have been at the point where the capstone has broken away underneath and created that 'chamfered edge' of which the 'hole' is part of.