Alison Steadman was great in her ex-husband Mike Leigh's screenplays - Abigail's Party, Nuts in May, Life is Sweet, etc. etc., and she has done a lot of other good stuff.
There are probably fewer comedy scripts containing older characters and less work for older actors to choose from.
I too found John Cleese's grating voice very distracting,
although I thought his acting was OK. The idea of the son coming back to live with his Mum is, I think, quite a good one and probably quite relevant these days, but the son's behaviour is so ridiculously childish that it isn't remotely funny, just irritating.