But there is an advance warning sign in the geographical layout we prefer, rather than "blue signs". It is about 100 metres from the turn.
https://maps.app.goo.gl/yp1cuCxWQeqCvPtq7For me, this makes for a weaker case on signage.
However I think the OP will be on stronger ground on PCNs issued between the date of the first PCN, and its arrival, and those issued between those two dates. Clearly the arrival of the first PCN was when you first realised the contravention, and how many times you had turned left there. Many councils will cancel these "middle of dates" ones provided the first one is paid, but they can play hard ball, I'm afraid.