Uk cap371:
As I recall STBY duty can be for max 12 hours or 14 hours? I cannot actually remember. Also a crew could be CTC for 24 hours, but the company would give them one hour (I.E. 1700-1800) when the crew member should be reachable, to notify a change of duty, etc. This is a good tactic if your going from say a late finish to a early start type duty the following day.
I always found the CAA at LGW to be extremely helpful on any questions in regards to CAA CAP371 of this sort.