If you record real time to any device you'll be going digital to analogue and then back to digital again, so it won't be the best quality...
To do it properly (and it would invalidate one's warranty if you were to be honest with them), get the HD out of the unit, into a docking station on your PC and use a program called 'Copy+' which will convert the stored digital info to a burnable format.
You can also clone your drive to a bigger one, and reinsert that, that will give you extra storage space for the future. You'll still be stuck with the wasted anytime space, but at least it won't be half the drive any more.
Some people run a SATA cable to a docking station at the back of the Sky box, so you can pop in drives like cartridges.
N.B. I have not done this yet myself, but I am considering it as an option
A link to the forum...
http://www.skycopyplus.co.uk/forum/index.php