I purchased the Humax HDR-1000s box after cancelling Sky and seem impressed with it so far. The only problem I've found so far is that the 4od app does not seem to fit the tv screen. It appears too large, and some other options at the bottom of the page are mostly cutt off on it's homepage, and then this continues when you go into a programme menu. All the other apps seem to display without a problem, and the normal Humax homepage menus all appear to display correctly.

Does anyone know what would cause this, and what I can try to fix it?

You could try a factory default to see if that ressolves it.
It's also possible your TVs overscan could be the issue.
What screen setting do you have your TV set at? Does it have a screen fit mode?