The bar helper recalls an African American soldier she met during WWII telling her that there were guppies in the wall of Taskerlands or maybe "duppies". In Carribean Folklore, a "Duppy" is a ghost or spirit, typically malevolent.
Writer Nigel Kneale based "Taskerlands" and the scientists who worked there on the BBC's research and development laboratory in an old country house at Kingswood Warren in Kingswood, Surrey.
According to Nigel Kneale on the DVD commentary, taping of the play went over allowed time so a number of the scenes had not been taped. The production was remounted but Michael Bates could not make the new dates, so his lines were given to Iain Cuthbertson.
Remade by Somethin' Else as a binaural sound play for BBC Radio 4, with script adaptation by Matthew Graham and Peter Strickland (the latter also directed). First broadcast on 31st October 2015, it had Jane Asher make an appearance as Jill's mother.
Several of the actors also have a connection to the BBC drama Doctor Who. Both on television and audio drama.