Given the general skankiness of the frame I'd expect to see dents in it if it was 531.
I've seen many a nice 531 frame with dings from handlebars, getting knocked or slipping when parked. I've even heard of visibly squished seat tubes from front mech clamps! (I've never seen that myself though)
If it's Columbus, which I read Mr Wilkin used a lot, then here's a table of tubing specs. http://www.equusbicycle.com/bike/columb ... schart.htm best checking the seat tube with Vernier calipers rather than tubeset weight unless someone else has a frame with the same BB lugs and dropouts to compare.
I'd think Columbus is fairly likely, but not guaranteed.
If it wasn't for the internal routing and Deltas on my SL-tubed Concorde, I'd strip it to compare. Sadly, the Deltas need a new cable every time you disconnect them.
Is the OD consistent immediately below the seat lug?
I think i'm going to get it powdercoated, build it up with the bits it came with (the frame got shed battered but all the components have never been used pretty much), a mix of 7 speed 105 and RX-100 and move it on again. I've had my fun with it.