The bike is a a 531 tubed Wizard, I just got an email from Mike Howard (one of the two builders) and he says that almost all of them were built for 27.2 seat posts.
The seat post I have now is not marked. I don't have calipers to measure it. Eyeballing it against a ruler it looks like the seatpost I have is 25 or 26mm, but that is obviously not precise.