These are soon to be superseded by a new Financial Services Ombudsman. They can investigate complaints on behalf of consumers and can make recommendations for fair compensation. They are normally a last port of call, after any internal complaints process has been followed. The British and Irish Ombudsman Association's web site provides a useful set of links to Ombudsmen who are members of the association. If the trader is a member of a trade association, there may be a conciliation, mediation or arbitration scheme you can use.
Some associations have both.
Do not be put off - such schemes are informal and generally inexpensive. But arbitration may not necessarily be cheaper than going to court under the small claims procedure. Check what fees you will be expected to pay if you lose the case. In conciliation and mediation, the idea is to enable the parties to settle their differences themselves. It does not result in a legally binding decision, though any resulting agreement may be drawn up as a legally enforceable contract.