I've double-checked my dodgy memory, and it was the RSL Christopher Bland Prize. From their website:
An annual award of £10,000 for a debut novelist or non-fiction writer first published in any form aged 50 or over.
History
The Prize was launched in 2018 and first awarded in 2019, thanks to the generosity of the family and friends of Lady Jennie Bland, in memory of Sir Christopher Bland who died in 2017. Sir Christopher is best known for his leadership in broadcasting, the arts and business. He didn’t start writing until after his retirement publishing two novels to critical acclaim: Ashes in the Wind and Cathar. His play The Easter Rising and Thereafter was staged by the Jermyn Street Theatre in 2016 – 100 years to the day after the uprising in Dublin.
In 2019 'Raynor Winn' was the first recipient, for The Salt Path. I hope someone connected with the RSL considers dusting off the 2019 shortlist and ensuring the other genuinely debut over 50 authors are brought to public attention if they've since fallen by the wayside.