Apologies for this, but I've done a lot of googling and the answer to this will get me going...!
I can create buttons to maximise windows left/right - but I can't seem to find the default icon for these in the icon library (i.e. the one shown in the trigger menu), and I thought that using the action would default to the standard maximise left/right icon of two vertical rectangles - one black, one transparent.
This will no doubt come up with trying to create buttons for many standard actions.
Are those icons included somewhere?