I think the switch to a single solo flight from three happened in about 1980 or thereabouts, roughly the same time that Ventures were introduced.
If you did a weeks course (sun to sat), you were bound to go solo quicker because of the continuity ('right laddie, just like you did yesterday').
I was a weekender so with 5 days at school between each session, you had to do a bit of revision plus sometimes you 'lost' a day due to bad weather (it was October) so it took me a total of 39; that last day I did 6 with my instructor plus 2 with the CO as check pilot then my 3 solos all in a Mk3 so each launch lasted just 3 or 4 minutes.