Mat,
It looks like you're using a ToolbarButton rather than an mmToolBarButton on this toolbar (I probably did that when you were at our office <s>).
It shouldn't make a difference (and I'll fix this in the next version of MM .NET), but for now, changing the toolbar button to mmToolBarButton should fix it for you.
Regards,
Kevin McNeish
Eight-Time .NET MVP
VFP and iOS Author, Speaker & Trainer
Oak Leaf Enterprises, Inc.
Chief Architect, MM Framework
http://www.oakleafsd.com