To clear up the manufacturer/garage warranty, could you tell us if this was a new car or second hand? With only a 1 year warranty, I'm assuming it's second hand. I'd also assume if an oil light came on in a 2 month old car, the dealer sould be the only place you'd be going, and you wouldn't expect to pay anything to have that resolved.
What Mathepac is getting at here is any warranty is in addition to your statutory rights. To pursue your rights, you must deal with the party that your contract is with, the seller. Going to another party and getting work done weakens your position, as the seller could claim the other parties work was sub-standard. You then have to prove that is not the case which gets tricky.
So even if a car is under the manufacturer's warranty, you should always deal with the seller. It makes your position for recourse to the legal system so much stronger.