What originally prompted my question is that I had Addictive Drums with kick and snare assigned to individual outs - a track each in Platinum. Meters were all fine (under 0db). When I panned the (mono) snare slightly off centre - the volume meters on that track went over the limit - I assume because of my pan law settings. However, shouldn't the CL on that track stop that occurring, as the PC was in Post FX mode?

I went and watched your video again and you basically answer your own question. If you notice when you have the volume fader at  0 db and adjusted the  gain fader for an increase the limiter did exactly what it was supposed to do.
It wasn't until you increased the volume fader that it appeared that the limiter wasn't working, in reality it was still doing its job but you increased the output after the limiter. Also since the volume fader is after the limiter you can turn it down to reduce overall volume, use the limiter to smooth out peaks and use the volume fader to reduce volume to the bus.

No, the track pan control is also post effects bin and PC , it is actually post volume fader. The easiest fix for what you describe would be to reduce the track volume fader, if you want to use the limiter stick it in there and leave the volume fader at 0 or lower.

Ooops, I need to read your latest reply first

Pan and volume are post fx.

You answered your own question again. The panning law did increase the output and since the pan control is post limiter it could not help you tame it. You would have to go back to your limiter and bring the limit down with the output fader to do it there or simply bring the volume fader down. Any adjustment you make to volume or pan is after the PC or effects bin.

Thanks teego and sanderxpanda. It was the pan which was particularly throwing me. I guess to tame any errant peaks which slip through (as happened in my original project) I would need to rely on the bus limiter to catch what had gone over on the track. Thanks for your help.

You are welcome but you can and should tame peaks at the track with a limiter. just leave the volume fader at 0 or less. I think if you go to the help file and look at the signal flow chart it will clear a lot of it up for you. Also as Bristol Jonesy said above you shouldn't be driving your tracks that hot , I think most people try to keep tracks peaking at around the-12 to -10 db mark  because all your tracks are summed to the master bus .