I'm thinking as he's stating to get better he needs to go back to his previous A's soon then. How do you feel about starting to up that first A another 15 mins for a few days then hold and see how long he naps for? I'm thinking you'll need to increase again at some point, but a little at a time is probably best if he's still recovering. Wdyk? I would leave that first nap uncapped then try for a CN at the same A you did previously and shorten to 20mins. When we were doing this we had 4.5hr A first thing for 1.5hr nap, 3.5 for 20mins CN then 2.75hrs to BT. Mine always has liked a monster first A though! He might get a bit grumpy at the time he usually goes down, but push through and do some low-key stuff. Marcia walks hers around outside in a backpack... I've just started doing this but I'm holding her.. Good for the arm muscles/bingo wings I suppose 
Thanks for the pointers Kellyjs - this is exactly along the lines of what I figured he needed, so was good to have someone else suggest the same.
Last 2 days have been a bit mixed, 1st day following the above went like clockwork and he went to sleep ok at bedtime (bit lively for first half of feed but then settled) and did almost an 11 hour night

but yesterday, although it looked like it should have gone just as well as timings were spot on in day, resulted in a total A time >5hours before bed and then he woke after 40 mins and I eventually had to feed back to sleep again as he got himself so upset.
No idea if there was something else going on? His 1st tooth broke through a couple of days ago but it doesn't seem to be bothering him at all any other time. It was very hot here yesterday and his bedroom gets the sun almost all day into the evening, so it could just have been too warm for him.... but his bedtime feed was a display of pure acrobatics and distracted-ness... so I think it was just that he wasn't tired enough

Total sleep over night was only 9 hours too... nowhere near enough!
So this morning he has been super tired, cranky and clingy and I couldn't keep him going more than 3hr50m.... he just crashed.
I think he'll sleep about 1hr30 and then i'm considering cutting the CN even further - to 15 mins?..... I just don't see how I'm going to get him tired enough for a reasonable bedtime otherwise? Last night was 9:30pm before he slept and then awake again at 10:10pm....
Or should I keep pushing the morning A and give the 20m CN a bit more of a chance?
Yesterday was brilliant! After two crappy car naps the day before, she actually slept in until 6.20am for a change, nap at 12 for 2hrs 15mins then BT at 6.30, fell asleep within 10 mins which is unheard of at BT recently....
Still going to push on with the set nap and BT as 2 naps just doesn't work for us anymore.
eeeek BT at 6:30pm is my worst nightmare

in this house that is a guaranteed 4:30am start! How old is your baby and when did you start doing the set nap + BT routine? Did it work immediately?