Thanks for the links. However, having followed them up I don't think the situation is all that clear cut.
Under UK and EU legislation electric bikes which provide power when you are not pedalling are only legal if they have something called a "type approval certificate". This is apparently costly to obtain, so most manufacturers fit sensors to their bikes which stops the motor if you stop pedalling.
However what makes the situation more confusing is that according to the A to B website, the Department For Transport have said that they understand that Trading Standards are "unlikely to take action" to prevent the sale of electric bicycles which do not have a sensor fitted or a type approval certificate. And Kinetics imply that they will only sell you such a bike if you are prepared to sign some form of exemption.