If your concern is about any part of the work being unsafe, highlight this immediately to the contractor, if the problem is not resolved promptly you should report them to Trading Standards.
If the contractor (or tradesperson) does not correct the problem or give you a refund, you can take your complaint to the trade association of which they are member.
Outline your rights and contact act in a letter, explaining the problem, what you have asked to remedy and what has happened in response.
Also, the Citizen’s Advice Bureau has letters you can copy that will show service providers you know your rights.
If no action has been taken to this point, you can consult a law firm or charity specialised in alternative dispute resolutions, where an independent person will look at your problem and decide what should be done about it.
Agreements made during this process are legally binding and it is cheaper than going to court. Should you end up in court in the worse case, some judges prefer if you have tried this route first.
Some trade associations have their own alternative dispute resolution, so try asking them before appointing your own.
More information about what to do if you have problems with building work can be found on the Citizen’s Advice Bureau website.