In my experience they try get out of it. I'm actually suprised they changed the tyres as they could of hit you with the old chestnut your mate had knocked the tracking out. As for taking it to an independant garage i don't think they would honour the bill it could be worth paying for an independant report but not repairs then consider either trying to resolve it or go to trading standards. I'm sure the warranty should be upheld for issues that arise in the first 3 months till they are resolved but new issues after the three months wouldn't be covered. Any parts replaced could have a manufactors warranty anyway with a bit of luck but the tyres won't be covered if worn due to tracking shocks or any other faults.