Can you help to identify these frames, bikes or parts?

^ seat-lugs look Nervex
e.g.

There's more pattern catalogue scans here (post #13 if you scroll down)
In addition to being a fancy foreign frame, there exists the possibility that 26.8mm can be a plain gauge British frame (e.g. 531 or 501)
To confirm this, stick a vernier to the outside of the frame tubes - if it's 1 1/8" (28.6mm) then it's plain gauge tubing, if it's 28.0mm it's Columbus (or perhaps French 531)

Clear as mud I know.

Thanks Rik, hugely helpful... I think the lugs may well be Nervex, the one in the image you've posted is remarkably similar, I think it might just be the thickness of the paint that's masking detail very slightly. If they are Nervex lugs, would this point at it being British?

I put the vernier on the tube and it's measuring 29.0mm! I think that's, again, probably down to the thickness of the paint, so I expect the bare metal would probably be 28.6mm you mention.

So all in, Rik, Ben, would you think this suggests a British built frame from the limited information I've been able to give so far?