It has a Nervex bottom bracket shell, and I suspect the lugs might be Nervex too as their pattern looks similar. I can't make out the number stamped on the bottom bracket (my close-up eyesight is poor) but it might be clearer when I've stripped the paint.
The rear dropouts have "...YCLO" on the left and "BENEL..." on the right, which is presumably Cyclo Benelux. The right one also has AD283 stamped on it.
The seat stays do not wrap over, but they're also not flattened at the seat cluster - they just taper quite narrowly.
The chain stays are only narrowly tapering (if at all) as they emerge from the bottom bracket shell, then taper more sharply just past the chain stay bridge.
The fork dropouts are unusually long and flare outwards slightly at the end.
The rear brake bridge can't really be seen in the photo, but it's straight and not curved.
The rivet holes for the head badge are 45mm apart (centre to centre).
The frame measures 56cm from centre of bottom bracket to top of seat tube. Including fork plus bottom bracket and lamp bracket (not shown, on the fork), it weighs approximately 3.1 kg.
I'm guessing 50s to early 60s, and that it would have originally been built as a 5-speed (there's no sign that there's ever been a front derailleur). I also think it's probably been quite a nice bike originally.
The seller told me someone had suggested maybe a Freddie Grubb, and I've found some images of frames with the same kind of head tube lugs (but with other things that didn't match). He also suspected it might be an early Viking (though he didn't tell me why he thought that).
Grateful for any thoughts, thanks in advance.
Alan