Thanks for the prompt and informative reply, it's really appreciated.
I'm using the SR04 simply because I have a box of them from previous workshops. But from the looks of it, I should accept that an analogue sensor is the way to go long term. Something to shop for in the new year perhaps.
I am currently using the Pro with a Pi2, which as you say, is a little large. Plan is to look a using the Zero with an Explorer pHat, though the lack of 3.3 might make the analogue sensor a no go.
So for now I think my demo will just be robot motors run unless obstacle is detected, regardless of distance, with buttons to start and stop the sequence.
- Neil.