Ok no probs, have sorted it.

For some reason i couldnt get a full mirrored subdiv proxy, after trying a few times, deleting history freezing transforms etc, it suddenly took and gave me a full mirroed subD, but it also had the other half of my proxy, i deleted one half, and found another poly surface that seemingly was just a ghost, no actual geometry, so i delted that too, and suddenly i'm able to use either split poly tool. . . Happy days