I checked in Device Manager and the computer confirmed it's got the most up to date driver - I checked by requesting an update,just to make sure. Still no more options on that slider.

The connection works fine, both for the picture and the sound (with an audio cable from the headphone socket on the laptop to a headphone input on the TV). It's just that I can't use the full screen. I suspect that there's a way to get TV to do it - but I've no idea what.