A double glazing salesman is generally a term used to talk about someone who spends a lot of time trash talking their competitors, since, double glazing salesmen often trash talk their competitors to justify their very high prices. John Paul Jones did do some trash talking about Robert and Page's "Unledded" project, particularly the fact that they played No Quarter, his "signature" Led Zeppelin song, without him. I think that Robert Plant is just taking the piss on John Paul Jones as a response to those criticisms, but I don't think he has much animosity towards JPJ as a person.