It's the ITV Hub App so it would be difficult to speak to the developer, it works on windows no problem so I think its more to do with how it interacts with the Android OS. The problem only occurs when you try to replay on demand services, Live TV is ok. Any help much appreciated.
Most android boxes (and some lesser-known phone brands) encounter issues streaming from more "formal" services, typically because the manufacturers don't opt to go for Google's Widevine DRM certification.
So you often either can't play these protected streams natively (as seems to be the case with the ITV Hub App), or you might be limited to running at a reduced resolution.
Third party streaming apps are really the main way to go here unless you're using a more upmarket device like a Fire Stick.
Unfortunately I can't recommend any in particular. Your best bet would probably be to google search "IPTV streaming apps for android tv box" and try them out at random until you find something you like.