Emissions are vehicle/date dependent Rich ie: a modern cars limit is different to say a 1970's model.
To be fair 1970's vehicles don't have the emission test at all - they just have a visual inspection which is left to the discretion of the tester. (I have a 1970 Series 2a Land Rover) - and yes I am being pedantic
They quite often do checks in a layby right next to a roundabout on the dual carriage way near here. I used to use the road every day and got into the habbit of getting straight into the fast lane as they only ever pulled stuff in the slow lane.