Hi Nigel,
Yes, I've looked at the factory photo and can't place the make.
I'd say from the style of the bike and that they all appear to be fitted with the CS Sturmey Archer gearbox that it is immediately post WWI. From the size of the flywheel I'd guess it is a Blackburne engine and the front forks are Brampton Bi-Flex.
In other words a bike assembled from bought in components as was common in those days but who knows what name it was sold under. I suppose I could plow through my copy of "The Motor Cycle Index 1913-1924" and hazard a guess but I'd prefer someone who knows to tell us.
Cheers, Bob