Thanks again DJ. Just to follow up on this, I couldn't quite get the exact behaviour without a pretty hard core CSS setting for my main content of "left: 0px!important" since it would squeeze the heading and main window when opening the menu. But after doing that (and handling the offset of the secondary window appearing underneath the header) it works a treat.
Will be interesting to look at how SAP did this differently to avoid this setting when I get a hold to a live Fiori launchpad demo.
Cheers,
Matt
