Back to the original question.

Ignoring the SPDIF link, what comes out of the HDMI socket when the box is receiving a UK HD terrestrial broadcast with surround sound?

With no downmix is it 5.1 AAC?
With downmix is it 2 channel PCM, 6 channel PCM or something else.

If I cannot get surround sound without spending mega amounts on new equipment, which even then may not support the MB "5.1" output directly, at least I have the option of spending another 60 quid for two satellite tuners and converting my box to a satellite (Freesat) PVR with HD capability - assuming the MB supports a 7 day Freesat EPG .