Thanks for putting the link up to the BBC documentary.
Had me immersed for several hours. Not just the beams but other stuff on magnetic mines.
Amazing what could be deduced from the wreckage of the German bombers and their radio equipment. The standard of work on the sets was phenomenal for the day. Watched a follow on about secret listeners. It would have been made in the 70s when the work of Bletchley was still secret.