Depending on the nature and seriousness of the complaint there is the Legal Ombudsman...
http://www.legalombudsman.org.uk/
If they work on the same lines of the former Solicitors Complaints Service they usually say to go through the firm's own complaints procedure first. There is normally a dedicated Complaints Partner and procedure and this should be detailed in the initial confirmation of instructions sent to you when, if it is the case here. Just trying to cover in case it's not, say, a situation where you are complaining about a solicitor you have instructed.
It does depend on the circumstances and the outcome you are seeking and what is appropriate in the circumstances (eg a complaint and appropriate redress, professional sanctions or more along the lines of legal action) as to how best to approach things.
The Law Society should be able to give some initial advice on how best to handle things and who to approach in your specific circumstances.