@Bim ******
financial requirements for a Non-O for the purposes of retirement, from the UK embassy website:
"Financial evidence e.g. A copy of pension statement if the applicant is a pensioner, or a copy of 1-month bank statement showing your income from pension, or 3-month bank statement of at least £10,000"
However, their website is out of date so it's best to contact them to check first if you want to go this route.
Having said that, I think these have been the requirements for that visa for some years.