It wasn’t ‘influence sphere’ per say but thinking of sphere led me to radius, it would appear radius (under ‘Force 1’) is the primary control for the shatter and keyframing that value let’s you put the shatter at any point in time.
Sometimes there’s just so many options to look at you just don’t think some of them apply to what you’re after, just takes someone else’s guidance sometimes, so thanks for putting me onto the right track 🙂