Gappie, you are a LEGEND. I've sorted it; I was looking in the wrong place (d'oh!) but it still seems ludicrous that an end-user has to hack xml files around to get something this simple to work. The diNovo Edge keyboard is a little quirkier due to the Play/Pause/Next buttons being a 'Function lock' command (ie. the F key is required to be pressed to make them active.
In case someone else stumbles across this thread, here's what I did:
1) Close down SetPoint if it's open
2) Navigate to C:\ProgramData\Logishrd\SetPointP\Devices\Keyboard\2000056\2000056.xml in Notepad (2000056 is the diNovo Edge keyboard xml file)
3) Take a backup of the file and save it somewhere else for safekeeping
4) Right-click > Edit and search for the following text:
<!-- Button Name => FLOCK F5 -->
<Button Number="786614" Name="F5">
<Trigger Class="ButtonPress">
<PARAM Button="786614" />
<TriggerState Name="ButtonDownUp" HandlerSet="MM PREVIOUS" HandlerSetGroup="Keyboard" />
</Trigger>
</Button>
<!-- Button Name => FLOCK F6 -->
<Button Number="786615" Name="F6">
<Trigger Class="ButtonPress">
<PARAM Button="786615" />
<TriggerState Name="ButtonDownUp" HandlerSet="MM STOP" HandlerSetGroup="Keyboard" />
</Trigger>
</Button>
<!-- Button Name => FLOCK F7 -->
<Button Number="786637" Name="F7">
<Trigger Class="ButtonPress">
<PARAM Button="786637" />
<TriggerState Name="ButtonDownUp" HandlerSet="MM PLAY" HandlerSetGroup="Keyboard" />
</Trigger>
</Button>
<!-- Button Name => FLOCK F8 -->
<Button Number="786613" Name="F8">
<Trigger Class="ButtonPress">
<PARAM Button="786613" />
<TriggerState Name="ButtonDownUp" HandlerSet="MM NEXT" HandlerSetGroup="Keyboard" />
</Trigger>
</Button>
5) Highlight and delete it (make sure it's exactly this text!)
6) Save the file and restart SetPoint - you should then find that the media keys control the player when MC is not the focus window
Gappie, thanks again for pointing me to the right directory. All sorted, happy Funkmeister again!