Kev,
That was not intended as a solution, I wanted you to provide more information so we could learn where to go from there.
In Control Panel > Appearance > Toolbars > Page ToolBar
you should see the new tiddler button and it would be checked, to show on the page (not behind the more button)
- Check to see if there is only one button with the + Icon and hide one and show the other if so (Should not be occurring)
Open the tiddler
$:/core/ui/Buttons/new-tiddler
Then delete it, this is a shadow tiddler, so if it was mistakenly edited the delete will return it to its initial state.
Let us no if it is still a problem.
Regrads
Tony