Thanks Bryanj. I've not come across the Oxford product in my search but have in fact just found a whopping great
padlock called Lox on Ebay. It has a shackle dimension of 72mm and is shown fitted to a disc, albeit on a modern bike.
I'm about to fire off a question to the sellers asking for the 'shoulder' measurement - critical because there's so little
room between the back face of the disc and the spokes, low down towards the spindle where the lock would have to be fitted. I'll let you know how I get on - might be following-up the Oxford!