my memory matches yours. Looking into timings is problematic but hey I like maths puzzles. I don’t have a spreadsheet collating 1000 plus lesson payments and I don’t intend to create one. However, For all my students bar one payments before and after the change from 15% to 20% are a multiple of 25 pence. For that special case payment to me is either £18.70 (15%) or £17.60 (20%). Rolled up payments are all either 25n +20 pence or 25n + 10 pence
Tutorful took a 15 % cut up from start until her lesson of 21 Jan 2021 and then took 20% for her lesson on Feb 11th and all subsequent lessons.
Her first lesson was on 17 Sept 2020. The same increase seems to have been applied to my other long standing students at the same time ( I suspect but can’t prove Feb 1st 2021)