'76/'77 Claud Butler Cavalier seat-post woes

I'm having trouble replacing the seatpost in the above frame.
It looks like the seatpost that was in there was too small so the seat-tube and seatpost oval/misshapen - measurements range from 25.9 to 27.1mm.
I bought a 26.8seatpost - didnt fit (I was told the size was likely for the age & tubing?)
Now bought a 26.6, which goes in about 2cm but then gets very, very tight (almost impossible to move by hand).
I've done lots of waggling around with the old post to try and get the seat tube round again. I can't tell if if it doesnt fit and i need a 26.4 or if its just ridiculously tight from being misshapen.

Does know anything about likely sizes for Claud butlers at the time?
Or have any experience/advice getting steel seat tubes round again?
Thanks in advance.

(it's this one: http://www.nkilgariff.com/CBcats/Cat_77early/Cavalier77.jpg)

A good workshop or any frame builder can ream it, that might sort it. Sounds well eggy though.

I'd guess the tube is likely to be 27.2 if 531, 27 or 26.8 if more gas-pipey plain guage tubes, given you're talking about an old ish claud b. Not sure what 'level' this bike was sold as. Basic tourer? As they were made by Holdsworth for some time, if you happen to be near south east london you could take it to a guy who could easily have built it, Winston Vaz.

Can you turn the old post a full 360 degrees when it's in the frame?

Yes i think basic tourer is about right - 'essentially a Holdsworth Equipe' - plain guage tubing with 531 forks. Cheers for the name, back down south shortly.

Yes the old post will turn, did lots of turns to try and get the tube round again (by eye appears reasonably successful).

Appreciate the input guys.
Any idea if it's possible that it's a 26.4 seat tube? (before I buy all the seatposts)