Thanks Pete - i know what you're saying. It appears to be running fine, although the revs at idle arent the smoothest i've ever seen (varies by around +-50rpm). Drives OK and i cant 'feel' a misfire, just this damn light has appeared!!
I have maintained this car for a couple of years now so know it quite well and the customer looks after it and hasnt had this light till last week so its not a 'normal' situation - its something that has just appeared so something must be causing it (what i mean is i doubt it is something like the ECU needs a flash update).

We dont have a coil tester at the moment but may have to look into getting one if this carries on! I think i'll see if the EGR pipe has any effect and then get down to scoping the coil triggers to see if anything isnt quite right.