Following a reinstall of Windows 8 Pro and setting a 24/7 overclock of 4.6Ghz I can now play the game with max details/PhysX High and 8xMSAA and although framerates still dip to 25 at times it's very smooth and very playable. HAPPY
EDIT - Don't know what sorted it but hopefully it will stay like this, it's a great game and some of the visuals are a treat. Also got Alice: Madness Returns and some of the PhysX details on that are awesome.