I've just got SkyHD and they supply a basic HDMI cable to link to the TV. Is it worth upgrading this HDMI cable? I have a Sony KDL-40W2000 LCD TV and a Sony RDR-HXD860 DVD/HDD Player, linked with a QED Qunex HDMI-P cable, which seems good quality.
At normal lengths (say 5m or less) then one HDMI cable is pretty much as capable as another.
It's a digital signal, so the PQ is going to change due to different cables. All a more expensive cable might do is reduce errors over long cable lengths.
100k+ Sky HD users out there, 99% are probably using the free HDMI cable supplied by Sky. You will have difficulty finding anyone on forums like this who has had a problem with the Sky cable, or found an improvement in PQ by upgrading it. In fact, you can probably find more posts where people did buy a more expensive cable then had to admit it made no difference.
A QED Quinex cable isn't going to do anything a much cheaper cable can't do.