From the Which? website
The scam: usually carried out by young men, who go door to door selling household products. They carry a fake ID and claim to be recently out of prison or on probation, and claim that this is a legitimate rehabilitation scheme.
The reality: the household goods are supplied by a man (traditionally from Nottingham, hence the name) who employs them. A group of young men are dropped off to work an area, and then collected by the same man later that day. The knockers’ role is to establish where elderly or vulnerable people live, and this information is then sold on to other criminals. If they come to your door, the advice from the police is to phone 101 to report them. However persistent they are with their hard-luck stories, do not buy from them.