Ben Squibb way back in 1992 with a time of 23:51.

Membership lists at http://bobgrahamclub.org.uk/index.php?page=members and record lists (including the youngest, oldest, etc) at http://bobgrahamclub.org.uk/index.php?page=records.

There's a list of the fastest teenagers (23 of them to date), only one is under 20hrs though the next was just 6mins over that mark.

The lists are correct to the end of the year prior to the current one, i.e. they are currently to the end of 2022, on 1st Jan 2024 they will be good to the end of 2023 and so on going forward.