Harry Monk:
I can tell you that you can’t use it on the Dover-Calais/ Dunkerque ferries.Journeys involving ferry or train transport
If your vehicle is being transported by ferry or train, the daily rest requirements are more flexible.A regular daily rest period may be interrupted no more than twice by other activities not exceeding one hour in total. This allows you to drive the vehicle onto a ferry and off at the end of a sea crossing. If your rest period is interrupted in this way, the total accumulated rest period must still be 11 hours. You must have a bunk or couchette available during the rest period.
I think this depends very much on who stops you, used it on Tuesday this week from Calais to Dover then got pulled in Holyhead on Wednesday the guy there had no problem with it, let go after 30 minutes and no donations 