There are two, date related, Diesel engine smoke tests - pre and post 1st August 1979:

The test station will need to be satisfied that Diesel engines are in good order and you will be asked to verify it's been maintained properly and cam belts have been changed at the required intervals. If you can't,  the test will be refused or if the engine sounds rough then the Diesel emission part of the test may be refused .

The engine oil temperature should exceed 80 degrees C before the test can begin which may mean the vehicle needs to be driven on the road. Getting the oil warm enough is difficult at tick-over (testing when the vehicle arrives is good practice providing the rest of the test is carried out the same day by the same tester).

Vehicles first used prior to 1st August 1979 must not emit dense blue or clearly visible black smoke for a period of 5 seconds at idle. Dense blue or black smoke under acceleration which would obscure the view of other road users will also fail (acceleration is from tick-over to 2500rpm or half engine maximum speed whichever is lower for pre 1979 vehicles).

Vehicles first used on or after 1st August 1979 must meet the limits prescribed when tested with a properly calibrated smoke meter. In addition to the metered requirements they must not emit excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users.

Once the engine is at the proper temperature it will be purged by raising the rpm to around 2500 rpm or half the engines maximum speed if this is lower and holding it there for 20 seconds. A check is then carried out on the operation of the speed governor by slowly raising the engine speed to maximum (the rpm should flatten out at this point if all is well). The engine must not be run at governed speed for any length of time. A calibrated smoke meter is then connected and the engine accelerated three times, prompted each time by the smoke meter. If the vehicle meets the required level after these three accelerations it has passed this part of the test.

If the levels are still too high a further 6 accelerations can be carried out but as soon as the vehicle achieves the requirements the test is stopped. The objective is to minimise the number of free accelerations of the engine.

The manner in which the accelerator pedal is depressed is important. It needs to be pressed quickly and continuously but not violently. The full fuel position should be reached within 1 second.