Its basically an annual check, so doesnt always tie up with a service i.e if your doing 20k a year your going to have a service at 12.5k before the year is up.
In my experience, Ford are very tight on this and if there is a stamp missing it will just get thrown out. Also if there is evidence a panel has been repaired before i.e been accident damagaged, however minor it maybe, they will say nope. To be fair if they can riggle they will!