Splitter won't work, you'll need a proper switch.

To be honest, plugging your TV into the DAV is pointless. The sound from the TV will only ever be stereo, and the sound in the programmes broadcast on your TV (ie, Freeview or Freesat) will only ever be stereo, so plugging into a surround sound set up is pointless as the DAV will only ever see a stereo signal coming in. If it has Dolby Pro Logic it can pretend it's 5.1 and take the stereo signal and output it as a fake surround, but it will sound awful and very very soon you'll not bother using it.

Personally, just have the audio as it was intended - either proper 5.1 DD/DTS or stereo. Simple as that.