Well the center electrode of the plugs should have a flat end with reasonable well defined edges anyway, did you check to see if the gap had opened up in comparison with the others?
When you say it looked worse than the others and sooty, that could just mean that its running a little rich at idle which could just be down to bad injector spray pattern, doesn't sound like a lot to worry about.
I'm beginning to wonder if I'm a bit paranoid now Tricky, the sooty looking plug was what I found during servicing last year, at the time I thought the plug itself may have been malfunctioning because the cars performance seemed to improve following servicing (although their wasn't any rough running noticed Prior to servicing)
This time I got concerned because the plug tip looked more disfigured or squashed down (only slightly) than the others (didn't know they were supposed to have a flatter tip). The electrode gaps on all plugs looked to be fine. I agree I might be jumping the gun here, think I'll run it for a few months and check it again to see how it looks. I'll also do a bit of research into checking injector operation as I don't know much about them (I'm an old school carb guy you see ha ha).
Thanks for your input though young man!