Do not be tempted to do business with anyone that calls on your door unannounced telling you that work is necessary. Always deal with locally recognised traders who you can find and go back to if a problem arises.
Ask for references and the names and addresses of other customers with whom you can check that the work was carried out in a satisfactory manner. Do not be tempted by deals offering to do the job for cash avoiding any VAT or paperwork. Reputable tradesmen will provide paperwork, put all income through their books and generally do not need to call door to door to find work.
Trading Standards operates an approved trader scheme through Trustmark.