Technically on the audio only episodes, two characters were.
Dr Fawn was taken over by Jeremy Wilkin, using the same voice he used for Virgil.
The World President was taken over by Francis Matthews. His voice was just Scarlet with an American accent.
I cant say why Tingwell or Maxwell left, but at least it gave Martin King, Gary Files and David Healy more acting work on the show. Healy easily became one of my favourites.