Big Grecian,
I have never seen anything in writing from the CAA. I was told by my contact at the CAA on the telephone that it had to be done in 24 hours unless there was some valid reason, e.g. aircraft tech problems, which delayed you en-route. He also told me that this was a change to the previous rule.
However, this was me telephoning him for clarification, not him notifying me of a change.
Without being too rude about the CAA (why not!) they are notoriously bad about notifying schools about any changes.
But don't get me started on that one ........
Linda Mollison