Did you notice that the Volvo's "daylights" were off? That's how I clocked an unmarked Volvo near home a few months ago. Sadly I clocked it a bit too late as I was done for 98.2 average over 2.2 miles..think it took hime that long to catch up.
However, if you're in the North London/Herts area watch out for unmarked Skoda VRs and a bronze coloured unbadged Audi A6 diesel, also unbadged BMW 535d's! These are not used by Traffic but by the police driver training school. Got "flashed" by the coverts on the Audi for overtaking him when he didn't accelerate out of a 40 up to the 60 limit (and I did). Ho hum.