Well I've done about 10 miles this morning and the creaking has completely gone.
Just the tick-tick-ticking of the SA hub (and my wheezing) as I barrelled along this morning.
In fairness to everyone else's comments, I tried everything else before finally deciding on doing something with the hinge. Took the suspension block apart and greased it inside and out, front and back, regreased the seat rails, checked the seat post clamp, oiled the spds and added a few more drops of oil to the rear hub.
Part of the reason for doing everything else first was because when I first tried attacking the bolts for the swingarm, a 4mm allen key wouldn't fit, and a 3mm allen key was too loose. I thought they'd used a weird size that I didn't have.
Once everything else didn't work, I tried again. The 4mm allen key just needed a bit of persuading to go in.
It was a little loose on the drive side - handily, as you can't access this without removing the front ring.
For such a vital part, I'm surprised that it's held on by such a small bolt.
I did put some copper slip on the bolt before refitting.