Drat, Blood donation records don't show which were successful! But the dates of attempts in the last 5 years are below, and I've cross referenced with Facebook (search "give blood", and posts by me) to get some earlier dates and some more Hb results from some of the failed donations.

16 February 2024 - Hb 112
16 June 2023 - too low
4 November 2022 - success
10 June 2022 ?
28 January 2022 ?
3 September 2021 ?
23 April 2021 - success
3 July 2020 - Hb 123
13 March 2020 - success
15 November 2019 - success
5 August 2019 ?
8 March 2019 - Hb 116 "happened to me once years ago"

Earlier successful dates: 18 January 2010, 28 April 2010, 16 December 2014, 28 April 2015, 25 August 2015, 16 December 2015, 17 November 2017.

Given I've never had 3 fails in a row I think 3 September 2021 must have been a success too.