Question Author
Thank you all for your very quick answers.
The reason I noted it was that I had just crossed at a double T-junction (what I call a 'dog leg) when this car came past; it seemed to come from nowhere. It hurtled past the car in front of me, pouring black smelly smoke from the exhaust, over the diagonal while lines and accellerated at speed.
Some time later I came across it the same country road where it was chugging along, and there seemed to be some kind of barrier (like in a taxi) between the driver and the one passenger in the back seat.
There was also some kind of yellow notice on the back bumper with 3 numbers on it (different from the reg number).
I did not recognise the make of the car, but the whole thing seemed very odd indeed.
Sorry if this is not very coherent. I had all kinds of scenarios in my head!