Well, spent as little money as possible and the rockhopper is back in service.
@Antstark karma'd me a 26.8 seatpost, I bought a cheap 30.4 to 27.2 shim on Amazon and my lbs gave me a couple of v brake shoes, as bizarrely I didn't have any spares.
The rusted headset bearing was stripped down, cleaned and regreased, and while it's not perfect it is usable.
(Yes, those seatpost numbers are right. The seatpost should be a 30.4 but whatever was done to the seattube means it's actually more like 30.0. The shim I bought is in really tight and takes a 26.8 post perfectly.)
So in the end, and having sold a specialized chainset for more than paid for this bike, the net cost of a perfectly usable pub bike is about £15.
I've stuck an 8 speed cassette on the back and the 9 speed shifters and mech seem to work ok with it, no I've I've adjusted the stops.
I've done that because I want to easily swap between this bike and my partners on the turbo trainer.