So I now set it up at home taking all the time in the world, and despite inflating, deflating, shifting, adjusting and inflating again I can't seem to get the bow equal on either side. This tells me the kayak won't cut the water evenly and so I'm fighting the sea rather than gliding through it.
These pics show my best attempt. You can see that while the starboard or right hand side bow is nicely rounded:

the port (left) is bunched up and twists the outer shell:

Feeling by hand, the bow aluminium spar seems to be square in the middle. Is the backbone too far forward? You can make it out about halfway through the skeg.

Another (maybe related) issue is the port chamber that runs along the length of the kayak seems to kink slightly inwards where the 2 chambers meet.
This summer I'm planning to keep the kayak inflated throughout and only adjust the final psi before each trip, so I'm keen to take the time to get it just right.
I use the AEC on open waters, in conditions up to wind force 5 with a fairly heavy swell, so any power loss I can avoid helps

Thanks for any ideas or thoughts!