You're right - that has just jogged my memory - I was travelling back from camping in Broadstairs just before that deluge arrived - Southend seafront was like a waterfall according to videos posted online,
Rainfall seems quite varied in this event which ran from 6am till just after 9pm here
Wanstead 22.2mm, Laindon 25.3mm, Woodford Wells 35mm?? Is Ron's station playing up? A station in North London recorded 38.5mm.

Interestingly St James Park, my closest official station, recorded 38.2mm - the highest 24hr total 6am to 6am. That is some difference, the rain band must have pivoted