Kerbdog:
They must have climbed in at Dover would be my defence.
I once arrested two illegals who had climbed onto the catwalk of an artic while the driver was taking a break at Cherwell services on the M40! Driver got as far as Towcester before he caught sight of one of them in his mirrors. He called 999 and kept going until Northampton, where I pulled him over and locked them up.