Contact your home buildings insurance provider who will guide you through the best course of action to take. Usually, a surveyor or engineer will be dispatched to make an assessment.
Often cases of subsidence can be handled quickly and effectively without requiring foundation repairs, but where foundation work does become necessary it can be costly and disruptive.
To give yourself the best possible chance of avoiding underpinning (or similar repairs) then it is in your interest to contact your insurer as soon as you notice a potential problem. Tree control measures might be taken, or leaky pipes surveyed and repaired, and in some cases little more will need to be done than some cosmetic repair/redecoration.