In almost all circumstances there is only one answer, take it to the main agent; it is by far your safest bet. Any other option is risky and potentially expensive in the long term. One always has to bear in mind that the car is under warranty and there are so many ways a warranty can be invalidated and after all the car is not yours it belongs to the leasing company.
It can be tempting to get repairs carried out locally, especially if it is damage you don't want to claim of your insurance; sometimes this is because taking into account the size of the excess, it hardly makes it worth it. On other occasions it can be because the company is reluctant to make yet another claim under the company policy.
In practice Insurance companies say all accidents and events such as a break in or accident should be reported, whether the insured is claiming or not. There was a case recently where the owner of a car had it broken into outside his house and the stereo system was removed. As the owner was reluctant to claim and affect his no claims bonus he repaired the car at his own expense.
A short time later the same car was stolen again near to the owner's home, possibility as a result of the police report, the insurance company learned of the previous incident when the stereo was stolen and refused to pay the claim. They claimed that if they had been aware of the previous break in they would have accessed the risk differently. Well would they? Probably not and one should never just accept these insurance company decisions not to pay out, they can be challenged and often are, successfully.
One of the problems faced by those who have work done at other than the dealership, is that should the leasing company become aware of this and feel that the work is not up to standard they can charge you to have the work redone. Bearing in mind that the vehicle will be disposed of at an auction; will it make any difference to the sale price? Well possibly because when a dealer bids for a car he will take into account the cost of any work that he will have to get done. In any event they are quite within their rights to charge you if you do not return a vehicle in a condition that is in accordance with its age and mileage.
Any repairs that involve mechanical damage are even more risky because manufacturers are very quick to invalidate a warranty, given the opportunity. With advanced diagnostics these days it is often easy for them to detect that work has been carried out on the car.
One of the exceptions to this is tyres. If you damage tyres, first check if you have maintenance included in your contract hire agreement and the terms of the maintenance agreement, because tyre damage may be included.
If there is no maintenance agreement or it does not covers tyres, they can be bought at one of the specialist tyre outlets, where they are likely to be a lot less expensive. It is however important to fit the manufacturer's specified tyre and not to be tempted to buy a "similar" tyre; it could affect your insurance.
In summary it is really the safest option to in general to get the repairs to your lease car carried out at the main agent, at least then you can be confident there will be no problems when the car is returned at the end of its term.