have you lost your a/c completely or just hearing a hissing noise from the dashboard? before you part with your money you can buy a cheap gauge in halfords for £29 and check the pressure yourself. if low, you can buy a canister of r1234yf (unfortunately £99) and top it up yourself. it contains uv dye and if you really have a leak, you'll be able to see it with any cheap uv lamp.
never trust the dealer, they love replacing parts you dont need