Aha!
This is actually working, I was making the rookie error of relying on the ‘preview’ function in Gutenberg. It seems this preview function for tablets and mobiles either doensn’t work, or doesn’t work with GeneratePress/GenerateBlocks – I’m not sure which. but suspect it’s gutenberg. – Perhaps you can confirm?
Either way, the results are expected when I acutally use a mobile device or use chrome in developer mode with the screensize restricted (instead of gutenberg preview)
Hopefully this will help someone….