To be very pedantic, the first season of JiS is missing, bar one episode, but it was remade in a slightly shortened version a few years later, and that version's complete. Well worth hearing, particularly the ultra-creepy Red Planet and the early X-Files-ish episodes of The World in Peril (the last BBC radio programme to out-rate TV).
The quick summary goes:
Journey into Space (A Story of The Future) - 18 episodes, 1953-54, remade in 1957 as Operation Luna - 13 episodes (the remake omits eps 1-4 of the original, and condenses the original eps 12 and 13 into one episode).
The Red Planet - 20 episodes, 1954-55
The World iN Peril - 20 episodes, 1955-56
The Return from Mars - 90 mins, 1982
Space Force (semi-sequel, originally intended as a continuation with the same characters) - 6 episodes, 1982
Space Force II - 6 episodes, 1984
Frozen in Time - 60 mins, 2006-ish
The HOst - 60 mins, 2009-ish
Oh, and ISIRTA is well worth a listen, particularly for Doctor Why and The Thing, and Professor Prune and the Electric Time Trousers!