Right! Hopefully the end of the saga.
The creak came back, but having eliminated everything else it had
to be the shim. After a bit of light prodding I was able to pull the shim completely out. It had obviously been epoxyed in place and the bond had been progressively failing over time. So, I scraped off all of the old resin from the shim and the seat-tube and refitted the shim with Copaslip. Creeeeak! Also, it was difficult to clamp the post without it slipping. Seems the thickness of the resin is enough to make the difference.
Anyway, I had a brief email exchange with Enigma and they said shims were notorious for creaking on Ti frames and the best thing was to bond it back in place. I gave a light sand to shim and frame, cleaned them up with alcohol, applied 2-part epoxy to both surfaces and then clamped it all in place with the seatpost inserted and left for 48 hours.
I rode all day today and not a squeak. Don't want to jinx it, but it would appear to be case closed.