Yamaha will only decline a warrenty claim if the dealer told them it could be self inflicted. So, in short your dealer is being an ass. Nail them with the sales of goods act, i've posted loads on here about it, have a search for it or try google, i can't point you as i'm on-line through my phone. In a nutshell, under the sales of goods act it becomes the dealers problem not yamahas......,

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Never underestimate the power of stupid people in large groups.