OK we can both stop looking now.

IN the prefs/audio tab there is a checkbox which, sure enough, is labelled 'RealTime Effects Enabled'.

Needless to say, it was unchecked. Funny I don't even remembering opening that interface lately.

Of course this is where I should have looked first but I didn't know of the existence of the box.

Thanks again for your time in researching this one, Mac.