Aha. I got it to work with Track Motion (instead of Pan/Crop).
But there’s one problem with this. Track Motion applies to the whole track – not just the clip. I expect to have several of these ‘call outs’ on one track and they could be anywhere on the canvas. So ideally, I’d want to be applying the transition to a clip, rather than the whole track.
So, I’m still looking.
I still think Pan/Crop should give me what I want (common sense), if only I knew what box I’m ticking that is making the animation centre the centre of the canvas rather than the centre of the image/clip.