There's a program called +Extract which can copy the DVB transport streams recorded on the SkyHD disk onto a PC. You can then use a program called Project X which will take those transport stream files and "demux" them into viewable MPEG2 files, provided the TV programme was recorded from a free-to-air channel (like the BBC channels).
If the programme was recorded from a free-to-view channel (like Sky Three) or a subscription channel (like Sky One), then the data will be stored on the disk in an encrypted format which so far has resisted all attempts to crack it, and probably won't be cracked in the foreseeable future. So basically, forget about trying to do that.
As for the space reserved for Sky Anytime, it is apparently fixed at 140GB. The standard disk on a SkyHD box is 300GB, thus leaving 160GB for user recorded material. Swapping the disk for a 500GB does, apparently, give 360GB for user recorded material, so you get more than double the space, even though the disk isn't double the size.
The maximum sized disk a SkyHD box can format is 500GB. It is possible to use an even larger disk, but you have to initialise in a PC using Copy+ before installing it in the SkyHD box.