This is a subject that has been buzzing around forums for a while.
This is an excerpt from the Cobra forum, he got an email from DVSA to clarify the situation (there was a lot of worried guys there with big old V8's):
"What we are trying to prevent here, is engines being changed for older variants thereby increasing the vehicles emissions since the time the vehicle was type approved as this is illegal on vehicles first used from 1 September 2002 under the Road Vehicles (Construction and Use) Regulations 1986. In the case of a kit car, these are often approved with an older engine are and will therefore still tested to those ‘old engine’ standards as the engine has not been changed since its approval (SVA/IVA test). Therefore, if a 2003 kit car was IVA tested with a 1987 engine, it will be MOT tested to the emissions standard applied at IVA.
Best regards Shaun Martin | Policy Specialist (Vehicle Testing and Roadworthiness)
Driver and Vehicle Standards Agency | Berkeley House, Croydon Street, Bristol BS5 0DA "
If there is nothing on your V5C front sheet under emissions then send complaint to DVLA asking them to send you a new one with this on
Hope this helps clarify