Finally I gave up on rebuilding individual modules and deleting certain directories, and did a full rebuild in order to get the buttons to appear - still can't find the cause of this.
Strangely - even though the code for the back and forward buttons is the same, the names of the images are equivalent, the back button appears as text in the toolbar, and the forward button appears as an icon.
However, you can see both of the icons in the 'visible buttons' menu.
I managed to do a debug build and I'm currently debugging the actual feature. Yeeey, finally! :)
