If you get the chance it's worth having a look at the original 1940 film, the one with Anton Walbrook and Diana Wynyard. It's closer to the play than the Charles Boyer/Ingrid Bergman one from four years later.
Scandalously, MGM bought the rights to the earlier film and tried to buy the negative and all existing prints in order to destroy them. So I'm all for the 1940 version getting all the attention it can. It's available on DVD and to stream from Amazon Prime.
Kudos to Johnny Vegas for this radio revival.