I'm guessing a bit, because it's possible there are regional differences in the technology, but...
I suspect your picture quality will already be limited by the DVDs you are watching and an HDMI cable will make little noticeable difference. Can you test it? Can you unplug the HDMI cable from your sky box, plug it into your DVD player, and see if you notice an improvement? If there is an improvement, it might be worth buying another HDMI cable.
A bigger issue for many people is the picture settings on the television itself. Sometimes the television will be set to widescreen, or set for a high-definition signal, while the DVD player or an old DVD is putting out an image that is just standard DVD quality and size. This will not look good on the television. You might want to find the picture or image-size settings on your television and experiment with those while you play a DVD, and see if you can get a more pleasing result.