After 'running around the houses', checking & double-checking everything, and even starting to doubt myself, the problem turned out to be an injector calibration, cured by using IDS (BIG thanks to Mark Carter for doing this).
Moral of the story is not to believe everything your scan-tool tells you. Despite doing loads of later model mondeos with my Snap-On it, apparently, wont do the early ones (although all indications were that it had done it). Made me look a bit daft but hey, at least i've learned something today and can chalk another one down to experience!
So all solved now and customer happy again.
