bullitt:Is the quality of 5.1 surround better through an hd lead or optical
This depends on a lot of factors. 5.1 is just the number of channels (i.e. 6 channels), not an indication of quality. So, even though an MP3 encoded at 128 Kbps and a CD are both in stereo (i.e. 2 channels), clearly a CD is going to be a much higher quality than the MP3. In the same way, what matters is the encoding of the 5.1 signal. If it's one of the new "lossless" HD audio codes (e.g. Dolby TrueHD or DTS HD Master Audio), these can only be transmitted by an HDMI 1.3 spec cable (or "category 2" as they are also known). Optical is not able to sustain the bandwidth required by these signals so can't compete with HDMI when these are involved. However, if it's an older standard "lossy" codec (e.g. Dolby Digital or standard DTS), whether it's transmitted by optical or HDMI shouldn't matter assuming you have a cable of reasonable quality to do this without relying overly on error correction. The key then is, can the device you're using transmit these signals via HDMI? In the case of most DVD and Blu-Ray players, the answer is yes. However, with something like the Sky HD box, this can only manage to output stereo over the HDMI cable, meaning you must use the optical / digital coax cable to get a 5.1 signal. This isn't a limitation of the cable, but the box itself. In this case therefore, you're better off using an optical / digital coax cable.
bullitt:also i have a sky brochure for the new deals coming out for existing customers, the sky hd box is advertised as having dolby digital 5.1 surround but doesn't say its through hd lead or optical, i find this very misleading seeing as its an hd box you would assume it has hd sound !
Potentially you find this misleading, but it's mainly down to marketing. HD sound isn't an official term really, it's just something which is used to describe something which is better than the current standard. In the case of Blu-Ray, HD generally means you'll not only get an HD picture, but also an HD soundtrack (since the space on a Blu-Ray disc allows this). However, it's important to note this isn't guaranteed and many early BD releases still used SD soundtracks encoded with Dolby Digital (Mission Impossible 3, Pan's Labyrinth and The Island being three quite big releases that did this from the top of my head). Sky HD is no different and, due to the bandwidth restrictions in place at the moment, I find it very unlikely we'll see a lossless HD soundtrack being broadcast on UK TV for some time to come.