Yeh I knew it couldn’t be done in the customizer, I just thought there may have been a php function that could be used.
However, having checked the blog layout again it already stays at a 2 column layout down to 768px, so should look fine on Ipads and smaller tablets. And also I’ve changed the number of posts shown from 9 to 6 so there is no odd post left, this was quite an obvious thing to do but it just hadn’t occurred to me until now. 😳
The homepage columns seem to stay at 3 columns at 768px which seems to work ok actually.