I owe the man from Bedshed an apology.
The key was an epiphany: not screw 1, but screw I. And then it all fell into place.
And I managed the assembly not only with Milo's presence, but also with his assistance.
As the mattress and bedding remain at Riverside the frame will remain charmless and naked for four more nights.
Please note: a whole post in my room, and I've not mentioned the floor.