Thanks, I've had some info from someone with the sane frame as mine saying a Thomson is too big. I did buy a cheap 27.4 post to try and it was really really tight. I'd still give one a try if I could find one at the right price, or another decent post in black or ti.
It feels like whatever I get may be an expensive failure. A couple of 27.2 posts I have do fit better than other 27.2 posts, but I'm keen to find the right post if I can, or maybe I'll have to investigate reaming the tube so a Thomson will fit properly (unless it will just slot in).