Dumb Coder has already given you a link to a website that explains your rights. The only thing that remains is how to execute the return without getting more grief from the dealer.
Though the legal aspects are different, I believe the principle is the same. I had a case where I had to rescind the sale of a vehicle in the US. I was within my legal rights to do so, but I knew that when I returned to the dealership they would not be pleased with my decision.
I executed my plan by writing a letter announcing my intention to return the vehicle siting the relevant laws involved with a space at the bottom of the letter for the sales person to acknowledge receipt of the letter and indicate that there was no visible damage to the car when the vehicle was returned. I printed two copies of the letter, one for them to keep, and one for me to keep with the signed acknowledgement of receipt.
As expected, they asked me to meet with the finance manager who told me that I wouldn't be able to return the car. I thanked him for meeting with me and told him that I would be happy to meet in court if I didn't receive a check within 7 days. (That was his obligation under the local laws that applied.)