It is not normally a good idea for the same solicitor to act for buyer and seller. It can easily lead to conflicts of interest - which it seems may be what has happened here.
You might have grounds for complaint that your solicitor did not keep you properly informed but, unless the market value of the house is less than a comparable one which has not suffered subsidence, it is not easy to see what you have lost. The only other possibility is that your insurance premium may increase once the insurer is aware of the underpinning - or you may have difficulty getting insurance.
Nonetheless, you are entitled to a proper explanation from the solicitor as to why he did not inform you.
If you want to follow the formal complaint route, go to the Law Society website to find out the procedure.