Did the Prime Minister
a) Not know
b) Make it up on the spot
c) Decide to back down but didn't inform the whips or Leader of the House
d) Tell a 'Brownie'
No doubt the truth will emerge during the course of the day. Maybe.
UPDATE 12.54: Apparently it's none of the above. Andrew Neil has just reported that the motion has been withdrawn. He thinks. Ferrets in a sack.