Just checking in to update in case anyone in the same situation has followed this - I had a lot of doubt right up until Saturday that this was working AT ALL, but Sunday, Monday and today my little boy seems to be getting with the programme. For a few days in a row, typically naps are longer and he either resettles very quickly for me or resettles himself in just a few moments after that startl-y 32 minute waking. I'm over the moon!! I'm sure this week will not be completely smooth and as you have pointed out, creations, having a little inconsistency to our days isn't helpful. However going with a shorter early nap and keeping the catnap until I can be more consistent seems to work for now. Thanks so much for the input.
To be clear I think what has worked is:
(a) resettling him when he wakes from a short nap, consistently - I haven't done this for every morning nap because these have been longer on their own, but I have literally camped in his room from 30 mins into the second nap to get this longer
(b) extending A times to around 2H40 mins...he seems to need this throughout the day except for before the catnap when he can be a little less tired (isn't sleeping for as long anyway)
(c) consistent timing of naps
Also insisting on him settling independently for a nap, returning to comfort/jiggle when he is crying but leaving the room otherwise...this relies on him not being overtired, so the nap timing is crucial. We had a terrible night the night before last thanks to some tummy issue (I strongly suspect a delayed allergy to something he ate or I ate) and keeping him up the following morning was a challenge, then he screamed into his first nap, so I had to settle him and it was a short nap. Second nap he settled on his own initially, and barely required my help to go back to sleep and make it longer when he woke after 30 mins.