One day, a man going along Ethelburt Avenue knocked on my door to say there was a problem with my downpipe. He was accompanied by a builder, who lived in Oxford. I agreed with the builder that the flat roof of my extension needed repair and engaged him to do the job. During the job, the kitchen was flooded whenever it rained. Payment was made for a stage of the work. Eventually I dismissed him and Trading Standards got involved. It would appear that the builder was not a crook, but trying to do a job that was quite beyond him. Indeed he agreed to repay the money. But this proved an empty gesture, because he went bankrupt. As for the man who brought the builder, he disappeared without trace.
And the moral this tale: only engage a local builder recommended by someone you know, or whose previous work you can inspect and never, never a stranger that knocks on your door.