My thanks to Sven – https://mathstodon.xyz/@SvenGeier/114819445178105464 – for reconnecting me with this which I saw a while ago but had been unable to rediscover.
I now know, courtesy of a post on @RichardJMurphy’s blog – https://www.taxresearch.org.uk/Blog/2025/03/26/from-centre-right-to-hard-left/ – that it was a letter to The New Statesman in 2023