When I copied the contents of my 300 GB Sky HDD to a new 2 TB one via my PC, I noticed there was a sizeable chunk of recordings that could be played on the PC. They were from free to air or free to view channels, I forget which exactly is the one without any encryption. Could be a way of freeing up a decent amount of space.
Otherwise it sounds like it'd be worth paying the bit extra for Sky Boxsets if you don't already. There's tons of shows on there that can be downloaded on demand so you don't have to store them locally.