If you suspect subsidence in your property then you need to contact your insurance company so they can send out an engineer to inspect the problem.
The problem may not affect the full building and may be easily treated but there may be more advanced work that needs to take place to rectify the subsidence problem.
Where cracks in internal walls have occurred such as in doorways, you should ensure that there is an easy exit to the room and property if your property collapsed. This is an extreme measure but if cracks are left for a considerable period of time then this may happen.