The fifth episode of the third series was broadcast on 9 December 1952, and saw the Goons embark on ‘The Expedition for Toothpaste’. The listing, with a suitably silly promotional snap, is available here.
This script was reused in the fourth series as episode 20, and was slightly renamed as ‘The Toothpaste Expedition’.
It was around this point that Spike Milligan’s mental health reached a low point, meaning he could not perform. He was replaced on alternate weeks by Dick Emery and Graham Stark from episode seven of this series. Before that, Sellers and Secombe manned the show themselves – Sellers taking most of Milligan’s parts (ahem).
Seagoon: I've always wanted big parts! Grytpype-Thynne: You always had them Neddie, you and Bentine.
(from ‘The Last Goon Show of All’, broadcast 5 October 1972)
The burden of writing fell solely upon Larry Stephens, with Jimmy Grafton supporting.
Quoted in Humphrey Carpenter's biography of him, Milligan explains that the breakdown happened slowly, and culminated in him deciding to kill one of his co-stars.
Ordinary noises were magnified in my brain until they sounded a hundred times as loud as they were, screaming and roaring in my head... [Finally] I thought, 'Nobody is on my side. They are letting me go insane. I must do something desperate so they will put me in hospital and cure me. I know what I'll do. I will kill Peter Sellers.'
(taken from Spike Milligan: The Biography, by Humphrey Carpenter, published Hodder & Staughton, 2004)
He did indeed enter Sellers' flat armed with a potato knife and declared his intention, and ended up in hospital.
In Carpenter's book, at times the author is quite cynical of the truth of Milligan's illness. While he cites Spike's unreliable memory as a reason for this, I find it quite unfair to be as dismissive as he is. Mental illnesses often present themselves as quite illogical to observers and it can be hard to understand the sufferer's actions. Trying to make sense of an illogical illness is always going to be in vain, especially this far removed from the period and the person.
Carpenter does present some more balanced views, however, and has obviously done some medical research in an attempt to accurately describe Milligan's symptoms. He describes this particular episode as showing that Spike was "both more and less sane than he himself thought" - defined as not insane enough to actually kill Sellers, but certainly suffering from serious symptoms such as paranoia and hallucinations.
It's impossible to know for certain when this far away from the person in question. For this reason, I've decided not to delve into this aspect of Spike's life or to attempt to 'diagnose' him. Too many people try to do this to others that they do not know, and it's not helpful.
The pain is too much A thousand grim winters grow in my head. In my ears the sound of the coming dead All seasons, all same all living all pain No opiate to lock still my senses. Only left, the body locked tenser.
('Manic Depression', written by Spike Milligan in 1953 or 1954)
Australian media company Virgo Productions has a Milligan legacy website here - appropriately titled 'I Told You I Was Ill' - and it has some more of Spike's poems about mental health.