One final piece of information, left here just in case anyone has this problem and are looking for a couple of other steps to check -
I took apart the Bang & Olfusen and re-entered service mode, only to realize that volume is not one of the settings on service mode.
Further checking of every part finally revealed what the issue was - I had bent one of the SCART cable pins when pushing it back into the TV. So just the sound pin on the SCART plug was bent. That was the problem. I've straightened the pin and the issue has been resolved.