I found out how to do it in the end by using losetup to associated it 
with a device, but in an unmounted state. I could then run resize2fs. 
Unfortunately I couldn't then work out how to shrink the img file :(
So, working from what you said Chris, I created a new img file from 
scratch and dded/copied stuff across. However I've now noticed there's a 
subtle difference in the two ext4 filesystems: the one has a flag of: 
unsigned_directory_hash, whereas the other uses signed. The other 
difference is that the one has a needs_recovery filesystem feature. I 
can't find out how to change/enable either of these options using 
tune2fs; does anyone know how to do it?
Leo
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