A sinkhole is quite possible. There are many hundreds in the Bristol area. Let me note that some sinkholes are stable for centuries and no threat to the buildings in and around them, because water is no longer significantly dissolving the rocks below. A sinkhole can therefore be lived with. One part of a sinkhole May be active, in fact, while another is stable, in the time scale we humans have to work with. But other sinkholes are a serious problem, and weter than average weather over a period of years can make the surface unstable at a rapid rate. Also, I should ask you to broaden your view about the changing surface. There is much limestone in Bristol, and some of it is dissolving in ways fast enough that you can see as an observant resident, so that causes some walls to fall in some places. But it does not necessarily mean there is a sinkhole there. It still might damage your house! So you don’t need a sinkhole to have a geological problem. I would bring in an expert, if you know one or know someone who knows one. I am afraid I am not an expert, and I do not know any. I do hope this helps some! Ed