spongehammer - lol - I guess I need to make my instructions slightly less unclear!!
Firstly, just confirm you're running demo v188.8.131.52? That's the one that's loaded in my dropbox folder and is the latest demo version. Once that's checked, on the start-controls page just press the key you wish to change (ie Up Arrow) and it should become highlighted and flash, then select the new key you want to replace it with... It should stop you from doubling up controls etc.
If that doesn't work, pressing Num 0 will delete the cache, just in case the old json has corrupted my rather poor logic. If you're trying to use Num 0 as a control key then that might be a problem... lol. I've just discovered that the number key sortcuts don't work in Firefox.... but they are ok in Chrome and IE. I love browser technology .