pomme pilot
With reference to the specific question of being LHR based and being called out for an LGW trip of home standby, It's worth emphasising the point Juan made - there is no time limit, you are simply required to use your "best endeavours" to report at LGW at the earliest opportunity. It's definitely a rule worth being aware of the first time you do reserve since it's just possible somebody might ring you up and try to persuade you otherwise....
Good luck on the course.