Psychiatrist Fulton Mackay tests Michael Crawford in the
Some Mothers Do 'Ave 'Em episode
The RAF Reunion (1973):

"Do you know what it's supposed to be?"
This is what they're looking at. I was as ignorant as Frank Spencer about the 19th century painting:
It's title is
And When Did You Last See Your Father? by W(illiam) F(rederick) Yeames:
From the
Liverpool Museums website's section on the Walker Art Gallery (where it hangs):
The painting shows an imaginery scene in a Royalist house during the English Civil War. The Parliamen-
tarian soldiers have taken over the house and are questioning the little boy about his Royalist father.
Much more here:
Liverpool museums - And when did you last see your father? feature
And
'An Alternative Interpretation' at
History on the Net.