What have the garage done so far to have cost£3500. I think that if you had taken it to a Mazda dealer you would have had a much better chance of them sorting it out. If the problem was the camshaft I am sure they would have given you some goodwill towards the cost as they know that the camshafts have had problems, but as soon as you go outside the Mazda dealership then any goodwill goes out of the window once out of warranty.
My car is also july2013 but I decided to take out the Mazda extended warranty at 3 years old because of this problem but I hope I don't need it. So far touch wood the car runs fine but I realise there is a chance that the cam could go. It must be remembered though that problems always seem worse on forums as people only talk about problems so what percentage of camshafts have gone I don't know but I think it's probably a fairly small percentage otherwise we would hear a lot more about it. I hope you get your car sorted soon without any more cost but keep pushing at Mazda uk for camshaft wear that shouldn't have happened. What mileage have you done?
