It may be his notes on the Cambridge Part III essay, which contain a lot of insights about mathematical writing that generalize beyond writing the Part III essay, naturally with his trademark wit: https://www.dpmms.cam.ac.uk/~twk10/Essay.pdf

I will always be grateful to Prof Körner for advocating that people who did Part III retroactively get awarded MMath masters degrees, thus making the most hardcore year of my academic life at least somewhat comprehensible to people outside Cambridge.

That's the one, thank you. Likewise about the MMath.