It looks like I spoke too soon!
I have still be getting shaft to rest contact, even after changing the nocking point. The contact was much more intermittent and it had moved from being about a third of the way up the side of the shaft, to being almost directly underneath. So the arrow wasn't dropping as low (presumably due to not sliding down from the nocking point), but it was still making some contact.
After a evening of fiddling around with things I found the solution - increasing the tiller seems to have fixed the problem.
I'm not 100% sure of why this should be, I guess that by weakening the top limb, relative to the bottom, the bottom limb moves further forward relative to the top limb.
I guess that prevents the back of the arrow from being pushed down by the top limb leading the bottom.
I don't know, just a guess, but it certainly worked.
I had been shooting with a tiller of about 0 to 2mm while fiddling trying to eliminate this contact. I've now set it to +6mm and it seems to have worked.
Not sure if it's relevant, but by setting tiller to 0, then putting the limbs on upside down, I got a +6mm tiller. So I guess that makes the "natural" tiller of the limbs -3mm (whatever that means)