27.0 or 27.2, either one is consistent with Reynolds 531 butted tubing, or (yes, there are others...).
Before we proceed, how's the paint? To my eye, on this ridiculously small screen, the paint looks bad. I'm thinking this bike is in for a total repaint. I don't mean to scare you; I could sure be wrong. But if I'm not, and if you are planning to do a complete repaint anyway, why not take off that existing paint (very carefully) and see what you learn in the process? you may find the shadows of the original decals or whatever, which will tell you more than any of us can.
Also: do I see a hole for an oiler on the top of the BB shell, left side? If so, that tells us something about the date (early 60's at the latest).
rhm,
The paint is bad, it has been repainted and put on fairly thick. The down tube has been sanded right down, so nothing will be visible there.
I'm not in a hurry to paint it, I just thought I'd ask here because theres so many members with so much knowledge
There is no hole on the bottom bracket shell, its a rust spot.