Hi Paul
I have a Tvix 6500 and a Clarke Tech HD sat receiver connected to my BV7-Mk3 only with HDMI cables. The sound is like it should. If I play a 5.1 movie or a 5.1 channel in the sat box, my rear speakers turn on. I don't need an spdif cable. Maybe you have to change some settings in the Sky box too, not only in the BV menú.
I don't have a link TV, but ML cable doesn't carry HD content, so you need to connect the Sky box with analogue cables too. This is what I think, but maybe I am wrong.