I believe that you can do it by using scart from the sky box to the dvd recorder. Only problem is that you have to play the program while recording the channel with your dvd recorder. What I would do is purchase a bigger hard drive and swap it with your current one if its not providing you with enough space. Its easy enough to do if you know how to use a screw driver you can swap them over very easily.
More details of how to do this can be found at the upgrading hard drive page http://satchat.co.uk/showthread.php?t=78