This is perfection – charming, eccentric, absurd yet oddly plausible – this could be the blueprint for the very best season 4 episodes. The plot is diabolical yet simple – doctors provide arsenic to would-be murderers (one client is described wittily as “an aspiring widow”), certify the death, and the body is then interred in a Cornish graveyard where the arsenic in the soil will disguise the true cause of death.

John Le Mesurier is perfect – reluctantly enslaved to his greed, there’s a hint of conscience and a touch of self pity, but insufficient confidence to get out whilst he’s ahead.

Cathy has a terrific fight with a cockney gravedigger (this, I think, is the fight where Honour Blackman knocked out her opponent during filming).

Steed gets to do some flirting in a cracker shop – and Steed’s chat-up has become far more charming than the crude sexism in the earlier series.