In my experience a main dealer 'approved' warranty purports to cover more things but in reality is as much use as a back street dealer warranty.

I've had to fight/argue/threaten to get items that are covered under an approved warranty with Toyota(gave up), Jaguar(won in the end), Vauxhall (50/50 cost in the end) and various other ones. Ironically when I bought a car from a car supermarket place I didn't expect them to cover a stone punctured air conditioning condensor, but they did as I noticed it within a couple of months of buying the car.

My impression is that main dealers don't give a flying fig if you're impressed with their warranty services as theres always another customer that will come along as they have little other choice if you want a new car, whereas a decent independent is hoping for repeat business.