Thanks for the replies guys...

As the man says, I am going from a bike currently running 700c (but knackered out old wheels), and looking for a fixed back wheel. I've already got a spare 700c front lying around, so it's a case of seeing whether the frame has enough spacing for the wheel to be able to move around in the rear dropouts (to ensure chain tension) without hitting the frame.

But from what I've gathered, I'm really not going to feel like I'm sloping forwards or any other silly old school lo-pro position, which is good!