Does your mum like Sinatra? If so, here's news of a new "play" that she might enjoy:
"Sinatra at the London Palladium, is a multi-media marvel, blending footage of Sinatra performing with a live 24-piece orchestra and a company of dancers and singers and opens on 8 March 2006, following previews from 17 February 2006."
I saw a similar thing of Elvis at Wembley about 6 years ago, and it was a highly entertaining night of nostalgia music.
But, you may be after more traditional theatre for your mum. If so, I saw that "Night of the Iguana" is opening in early December (I think at the Lyric Theatre), but can't remember who is in the cast. Might be worth looking into though.
Also Richard E. Grant is opening in a play at the Criterion Theatre, and there's an Ibsen play opening at the Lyttleton - both open early November, so there's no way of knowing critical response at this point...
Isn't Mumsnet funny? I saw the Sinatra item, and thought "oooh, must tell Thombat!?"