Right, quick update:
I have some MTB wheels, which are 26 / 559, but they are 1.75, 2, and 2.25 wide, too wide for my existing forks at the top of the forks; they aren't QR hubs either.
However, when trying to fit them into my forks to check on the drop of the brakes, therein lies another problem:
The hubs on the MTB's are 100mm wide, whereas on the wheel I am trying to replace, it is only 90mm wide. I can't bend my forks enough to get a hub that is 100mm wide to fit!
Answer is that an MTB won't fit.