Found and fixed the issue - SBR give this a go on your 430!
Looks like I won't have to get the dremel out after all. I've cracked it!
You were completely right jmone, Enabling Aero was the answer, my temporary problem was that I had set madVR to 'disable desktop composition' which meant that it was disabling Aero! I didn't initially make the connection between Aero and desktop composition. So unckecking that made a huge difference, the present times are now below 0.2ms. And no frame drops for those files in full screen windowed mode.
So, to sum up my madVR settings are: (in case anyone else is having problems with framedrops)
Set Windows 7 to use an Aero theme in Control Panel -- Personalisation
MadVR settings: decoding - all off
scaling - fettle to your hearts content (wouldn't suggest something with 8 taps though!)
general :
windowed mode settings:
trade quality for performance: none
Many thanks jmone for making me a happy bunny! I'm going to crack open a bottle of Aussie Shiraz in your honour!
Now to go and watch some stuff and stop fiddling!
SBR