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Nice to know these idiots get there just desserts.
And, Loosehead, I have had that happen to me - I was doing 85 on the M5 near Bristol and there was a dark coloured saloon behind me, keeping pace, I felt pretty sure that it was an unmarked car, and the driver must have noticed me looking in my mirror as he turned on the lights in his front grill, turned them off again, and disappeared off towards the M4.
However, in my defence, it was early morning, and we were about the only two cars on the motorway.