Under the Sale of Goods Act 1979 (as amended) you are entitled to receive goods matching the description applied to them. Therefore if it was described and sold as a 16v in the advert and invoice, that is what should have been supplied.
If, when you viewed the vehicle at the dealership, your inspection should reasonably have revealed that it was in fact an 8v, then you may be struggling to show that you relied upon the description. However, within the first 6 months following purchase, the burden rests with the dealer to prove that this was the case. As you obviously didn't notice the discrepancy at the time of purchase (and by the sounds of it, the average person probably wouldn't realise the difference from a quick viewing either) you should be ok on that score.
Presuming the date of purchase was within the past few weeks (and that you have not sat on this nasty discovery for a while) then you will be entitled to reject the goods and claim a full refund.
Also, under the Consumer Protection from Unfair Trading Regulations 2008, it is a criminal offence to engage in an unfair commercial practice which is a Misleading Action e.g. to apply a false description to a motor vehicle. On that score, if the dealer is unwilling to budge, the matter should also be reported to your local authority Trading Standards service via the Consumer Direct telephone number 08454 04 05 06 (who will also give you further advice).
Hope this helps.
