My shut off valve to the downstairs comode is not allowing water to fill the cistern. I've tried turning it previously it acted kind of quirky and difficult to move, and in one position it closed the water off completely until I cranked it a little more to the left. My guess is it may be clogged with sediment. It is only a 5 year old house.

Are you quite sure it's the valve and not the ballcock? Since the valve are so cheap why not simply replace it. If you do keep the old chrome nut and feral and reuse them. If there still a problem after replacing the shut off valve then let me know what type of ballcock you have, (seem image) Good luck, Tom