Have you tried it say over mains with the battery out?
You could also remove the memory cards and batteries/mains lead and hold in the power button for a bit.
Otherwise, post in the mobile devices bit rather than advice, someone might be able to suggest something else there.
You could try taking it back to where you got it from. Although it is 14 months old, and outside the 12 month warranty, the Sale of Goods Act says that goods must conform to contract for a "reasonable time" after purchase - whilst this is not defined, you have rights for up to six years depending on the purpose of the goods and their value. As a sat nav is quite expensive (I couldn't find one cheaper than mine at £130) you could say that 14 months was not a reasonable time, you do have to prove it is faulty though. Above all, they might have an idea how to sort it, and/or put you in touch with a repair centre if they won't do it themselves.