"He's not your dad. We never knew who your dad was"

I thought of this old television commercial a while ago, but I've only just found it because I had convinced myself it was advertising Viennetta.

What I also found, thanks to a Guardian article, is that Alan Bennett praised it in his diary of 2003 for the London Review of Books:

30 August. A commercial for Carte D’Or ice cream I would have been very pleased to have written. A family which includes the aged grandmother is having Sunday dinner. ‘Pass your father the potatoes,’ the mother says to the grown-up son. ‘He’s not your father,’ snaps the grandmother. ‘We never knew who your father was.’

There is an awkward silence, then the mother ushers the grandmother from the table saying: ‘Come along, mother, I’ll take you upstairs.’ On the way out of the room the old lady passes an open piano on which (this is the stroke of genius) she suddenly hits a petulant discord. It lasts all of a minute and is worth a dozen pages of dialogue. Why it’s advertising ice cream I’m not sure.

But, says the Guardian, despite Bennett's misgivings, the commercial helped double Carte D'Or's sales.