Not true.
If stopped and tested at the side of the road by the police you will be fined
http://www.roads.dtlr.gov.uk/consult...ons2001/07.htm
"It is an absolute offence under Section 42 of the Road Traffic Act 1988 to use (or cause or permit to be used) a vehicle on a road if it does not comply with the relevant provisions of the Road Vehicles (Construction and Use) Regulations 1986, as amended. The offence does not depend upon guilty knowledge and therefore it is not possible to mount a defence on the basis that a vehicle user did not know that the vehicle was in contravention of the Regulations at the time they were using it on a road."
"You (as named on the Notice itself) have been issued with the Notice because you were the user of the vehicle specified on the Notice when it was found, during a roadside enforcement check carried out by an Authorised Person, to exceed the maximum emission levels as laid down in Regulation 61 of the Road Vehicles (Construction and Use) Regulations 1986, as amended."
however...
"The Regulations permit the Fixed Penalty Notice to be waived completely in cases where you can show that the vehicle had passed an MOT emissions check within the preceding six months and the emissions performance of the vehicle is corrected within 14 days. To confirm this you must present both MOT test certificates (one dated before, but no more than six months before, the date of the offence; the other after, but no more than 14 days after) to the address shown on the Fixed Penalty Notice."