I've been in contact with Nic Henderson of the Bike Brothers website and he concurs that the frame may well be from around the early '70's, perhaps a bit earlier, or perhaps a little later. Built in the basement of the old shop (they moved out in the late 70's). The fork crowns are Cinelli, which doesn't help in dating since the design seems to date to the '50's.

I've been de-rusting, spot priming painting and polishing the frame, and have some 27" rims with a sturmey archer 3 speed hub, some drops and a Brooks. Still some bits to get, but it's not far from roadworthy.