The problem is the lorries have "Limiters" fitted, by law, and thus in theory they all have the same maximum speed. Of course this doesn't quite work and so Mr TNT thinks he can get an extra 1 or 2 MPH out of his vehicle and tries to overtake Mr M&S.
Mr M&S meanwhile is on a tight schedule and will not ease off his accelerator ~ ergo the time it takes one to over take the other is long and tedious for anyone trying to pass them both.
Best answer I have found is to pull off the road and take a coffee break.