Thanks, Andy (and happy new year - I just got back from my holidays).
I think your suggestion might suit some who come looking for a similar
solution, but it's a bit messy for me because:
1). The padding can emulate left justification for the RHS buttons but
is fiddly
2). I wanted to group certain buttons together, which this doesn't do.

In the end, it's easier just to add the buttons without any boxes so
that the first two are vertically adjacent, the bottom button is below
that and then the next two buttons are automatically in the next
column, also vertically adjacent. It seems a pity that there's not
better control over the placement of the buttons.

Thanks again

Paul