Thank you Barrychuckle and all who commented. Tangentially, I have to say that the way the writer retired Nora to Australia to care for her sister was an elegant way of keeping Nora in the show when she unfortunately could never return. She was a prime mover in the show and could not just disappear without explanation or ceremony. Putting her Sister Stella into Nora's digs and having Stella report out on Nora at coffee mornings, wiping down the banisters, discussing Nora with Alvin, etc. kept Nora in the audience's mind and in the show. The real question is whether Nora brought bleach with her on the plane!
In 'Of fish and Funerals', the neighbor Gloria, who was immigrating to Australia, pointed out to Nora that they don't scrub the steps in Australia. I wonder how Nora adapted to unclean steps. I bet she went out and scrubbed them anyway, in defiance of their local mucky habits!
The only other circumstantial character retirement I can remember was in 'The Man from Oswestry' when Clegg read Blamire's letter explaining that Blamire was now under the "skillful ministrations of a certain Lady", and introducing Foggy into the mix. A newly married man would no longer be a fit with the trio. Anyone could see that. It was a fait accompli in a few minutes of airtime with no rough edges left behind. Nicely done!