You do realise that not everything is covered by warranty? This is particularly important around "consumables" or things which would ordinarily be replaced/sorted by the owner eg lights, tyres etc. Also, if it's something that has been damaged/caused bythe owner, it won't be covered.
Having said all that, I had an issue with my clutch. One dealership rang me to say they couldn't replicate it and wanted to charge me upfront for the cost and would refund if somethig was found. Took it to another dealership and they diagnosed the fault through what I told them without looking at the car, ordered the part in and booked in the car. Didn't have to pay a penny (other than petrol to get to/from there!).