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We ask client to seal to trailers before departure and ask them to report any vehicle that has not applied his safety devices before departure. We would also be grateful if the Border Agency would inform us if they encounter one of our vehicles with security issues.

The Border Force might not like this answer I am currently in negotiations with the Border Force with the intention to make this reporting a part of their control system.

This works 2 ways, it will improve your standing with the Border Force and it will enable you to deal with the drivers who ignore your instructions.