Seeing as you are struggling with emissions at MOT time , and starting problems i would get a lambda sensor , emission failures are usually caused by a knackered Lambda on a 2.3 . But still change the plugs or at least check them to see how well they look Thanks for the reply. Mot tester reckon iI needed a new cat, had the a new one fitted approx 3 years ago a porcelain one apparently to get it through last couple of mot's they have put in a fuel additive and booted it as it was failing test on fast idle. Is the lambda sensor difficult to replace?