[robsterpaterson]

Here's the actual legal aspect which a couple of folk have almost got totally correct.
Any warranty offered by a dealer is in addition to your consumer rights. Any defect identified within six months of purchase is assumed, by law, to have been present at the point of sale. You don't have to prove it and the dealer can't deny it.
You HAVE to give them one attempt at fixing the defect(s) at NO cost to you. If they refuse or fail to do so then get that documented.
You can then get it fixed at a garage of your choosing, which you'll have to pay for, and then claim it back - through small claims court if necessary.
This all works. An independent dealer had to stump up £1,200 plus for a car of mine when they refused to fix it free of charge.