I have just been looking at the Sony HT-DDW890 manual online (that's your HT set-up, yes?) ÿI think I have the answer!
If your blu-ray player is connected via HDMI only, it will only accept stereo sound, which can be routed to the TV via another HDMI. ÿIf you want surround sound, you need to use an optical cable too. ÿThis will enable Dolby Digital and DTS soundtracks - your Sony HT doesn't handle the most recent High Def sound formats, so optical will work fine.
I've read what you say about using the optical input already for Sky+HD, but the diagram on the manual indicates clearly that there are TWO optical inputs, the second one specifically for blu-ray. ÿAre you sure you only have one, or do you need to have another look???!!! ÿIf you really do only have one optical in, you can buy optical switchers (Amazon have one for ÿ£3.49) - make sure you buy a switcher (2/3 in - 1 out) NOT a splitter (1 in - 2/3 out).