I'm mostly listening to stuff on R4 Extra these days, which is largely repeats.
Currently enjoying Rubbish, a sitcom about a man working in local council waste management department. Whimsical in that very Radio 4 way. First broadcast in 2007.
Another new-old discovery is The Pin, a comedy sketch show by a double act who I'm aware of as stalwarts on the comedy circuit though I don't think I've ever seen them live - I'm sure I would remember them if I had. They're very funny. I was listening last night in bed and annoying my wife by laughing too much (mostly at a sketch about Frank Lampard recording an advert for Gilette and being unable to rein in his urge to go off-script with some improv).
Also the repeats of the very first series of Ed Reardon's Week, which are reminding me that the early series were much funnier than the later ones.
And the Charles Paris mysteries - about a hack thesp who has a habit of getting embroiled in murder cases - recent versions with Bill Nighy are good, but there are also older ones, recorded in the early 80s, I think, which sound horribly dated now.