I spoke to him specifically about this crack and he said that it is not subsidance. He said it is an expansion crack at the weakest point of the wall and it will expand and contract periodically so needs a flexible sealant otherwise it will keep cracking. He said he would be surprised to see a clay brick house of this age without similar cracks. He said there is no damage internally and it is the internal walls that support the structure of the house so he doesn't think it needs any further investigation