I think the play was introduced as a way of keeping Alan and Celia more in the picture as their Last Tango seems to have settled into marital humdrum bliss.
Though why on earth they would want to live with Caroline beats me - newly weds, even elderly ones, not just living in the granny annexe but actually now living in the same grotty house?
How come Caroline's sensible older son who went off to uni has turned into such a drip?
However, I will continue to watch it, Derek Jacobi is, as usual, a delight and I think I mentioned on another thread the classic line which must be one of the best so far when Celia is trying to persuade Alan to take a part in Blythe Spirit:
'Me act? I can't act'
Wonderful!