Thanks so much both of you.
I think I am coming to the same conclusion - DS just can't do 1 nap yet. On 1 nap days, he'll do an 11.5 hour night, but that still gives him an early wake up as his 'cycle' ends up being 23.5 hours! And yes, I think trying to push the nap is getting us entirely nowhere!
So, I'm going to try the mini cat nap thing in the morning to take him through to a fixed nap somewhere in the middle of the day. He won't sleep in the car, but I think he'll be OK going down in his cot - won't like being woken up though! I may have to have the raisins ready...
I'm thinking about:
WU - realistically between 5:30-6:00
Catnap - 10-10:30?
But I may have to play around with this.
Nap - 12:30-2pm
Bedtime - aim to be asleep by 6:30.
The only issue is that with 2 hours of sleep in the day, he'll probably only do an 11 hour night, which would make his WU time at 5:30 still when I really want him to stretch to 6am. But maybe I can put him down a little later for bedtime gradually, so we shift it?
Thanks so much both of you - DH is sick of sleep talk so I'm so grateful!