So is a staircase / stepped crack along the motor , and the unlevel concrete interior floors definitely subsidence ? Or the walls because of sulphate attack on the floors ?
In Cornwall, when buying a house you have to have a search done for old tin mines, this involves trawling through the old mine records. The problem is, privateers , those buggers tunnelled all over the shop and avoided records. Heard of a few people disappearing down holes in the garden never to be seen again.
Built on a mine, I would walk away. Mortgage may be hard to get, insurance would be higher, there could also be health implications. Compensation is available, but not sure it would be if you bought the place knowing about the issues. It has already been said, walk away.