[Si-Co]

Presumably they were responsible for either editing it or playing it in (though admittedly I'd assumed imports were played from Central... but maybe some weren't).

I can’t speak for every genre of imports, of course, but those whose clocks I have seen from the 80s generally showed NFC or ITFC (the “library” that did the initial edit and was responsible for physical distribution of films and purchased programmes to the ITV stations for playout). If for whatever reason a VT machine at ITN was used purely for playback, then that wouldn’t be reflected on the VT clock. So we are left with the speculation that it may have been edited at GIR/ITN for some reason, and their name was accidentally or purposely left on the clock. I’m far from confident with this theory though.