Right Loats123,
You said you have an optical cable plugged into your sky + (u r lucky as my older one doesn't). That means you have a digital link to your HT554PH and so if the film etc on sky is broadcast in dolby then you can get the audio in 5.1 (5 speakers and sub). Usual sky and freeview tv is broadcast in stereo so you will only get sound through the front two speakers in that case.
As you say you have a HDMI cable from your HT554 to tv, you therefore u have a digital audio link/picture which is best. Set your input on the HT554 to TV using remote top left button but again it will only be front speakers in stereo unless a program is broardcast in dolby.
A scart lead is an analogue signal and so I think you will only get stereo with that connection. Note though if both scart and optical is plugged into the same unit picture is by scart and audio by optical but you would have to change the input on you HT554PH to OPT AUX.
When playing a dvd you will allways be able to get at least Dolby Digital 5.1 but note the rear speakers only become active when needed for the surround sound effect so won't be on all the time.
Most dvd's have 5.1 as a minimum but some have the option in the dvd set up to change from 5.1 to a slightly higher standard called DTS but look on the back of the dvd box to see if it has the red DTS symbol.
By the way sky visual HD is only broadcast in entry level 720p and not full HD which is 1080p. The only way you get full HD is through blue-ray or playstation. I've got a 42" regza 1080p.
There is the best HD sound when playing through a HDMI cables called Dolby true/master but our HT554PH won't play this as no decoder for that.
Now for my bit!
If you have a dvd that has DTS audio on the box can you play the dvd and go into the dvd set up option and change the audio to DTS and see if sound is still on?
I have rang LG again and they insist it has a DTS decoder so it should play. They advise going back to Currys with a DTS audio dvd and try it in the shop.

I might just have to settle for the 5.1 but its just the fact it should play that niggles me and not that is it that much better that normal 5.1.
Hope that helps you anyway!