Thanks EAW, i’m actually doing some motion graphics stuff so only have emitted light, no lights or reflections.
Since i wrote my post i discoved the add-on " Animated Render Border" which does exactly what i need, you can keyframe the bounding box size and position to track your object. I’m using motion blur with very long shutter times, so need to tweak this (although the add-on can automatically track your object which would be the easier method if you aren’t using motion blur or particles or whatever).
I’m testing it now and am managing to render a small object moving around the screen approx. 50 times faster than without the addon.
And it saves a huge amount of disk space!
Fingers crossed it stays stable and the developer keeps supporting it!