If selling it within the family or friends circle then an erase of all sensitive info and an erase of free space with a programme like
Eraser should suffice.
If you are selling to an unknown then for the sake of £30 it might be better to replace the disk. My laptop takes 5 minutes tops to replace the the disk. I'd rather do that than go through the erase hassle.
The last one I sold I replaced the hard drive with one from an old work laptop. No personal information on it anyway (and nothing sensitive from work either).
Mind you I replaced the hard drive more for ease than security.
I'll leave the paranoia to others....