This I would think:
I would be grateful if anybody could help identify the following film. I saw it once, in the early seventies on TV.
British black comedy from the 1950's or 1960's. The plot concerns a poisoner who I think invents his own poison that drives his victims briefly mad, before they drop dead. They behave in a reckless, uninhibited manner. For example, a very timid, mouse like woman, goes out marching confidently with a placard in support of women's rights, before dropping dead. There are a number of people the poisoner wants to bump off, (reminds me of "Kind Hearts and Coronets" a little) for reasons I can't recall.
One of the victims ingests the poison through sucking the end of their pencil, which the poisoner had contaminated, knowing the victim had this habit.
In the end the poisoner himself is accidentally poisoned by, I think his wife. Before he goes into his mad phase, he realises what has happened, and says calmly to his wife something like, "You will have been the cause of my death".
As I remember it, the film has an eccentric feel, a little like an episode of The Avengers, which I suppose it might have been, but I don't think so.
The mists of time may have distorted the fragments I remember, but if you've ever seen it, I hope this will be enough to jog your memory.
I'm sure that's it!
Thank you so much for solving it for me, and you responded within half an hour of my post!
Solved after all these years! Many, many thanks!