Gentlemen, thank you all for these replies. Very informative.
I've just put a new set in (at 90,000) so I will check at 110,000 and if all looks well, check again at 130,000. (I service at 10,000 intervals - it's easier than trying to remember than 12,000 intervals - also means if I do go over my 10K interval, I don't have to worry for a couple of thousand k's)
Now, anyone want to buy several sets of hardly used ST1100 plugs??????