What year is your car? If it is 2001, as suggested above, then it will have been through SVA and therefore will need to pass an emissions test. Some MOT testers may not understand the rules correctly and let you get away with a no visible smoke test.
MOT emissions test flowchart.
The full manual is here - section 7 covers the emissions test and includes the flow charts.
Edited by - Shaun_E on 7 Feb 2012 08:23:38