I’m looking at selling one of my older laptops (since I upgraded), and want to be sure that all the datan has been removed and can’t be retrieved (my business accounts etc.).

Whilst looking for a solution I stumbled across DBAN on the SourceForge.
DBAN (Darik’s Boot and Nuke) looks spot on the money, mainly because I don’t have to boot into an Operating System before it will wipe – but a word of caution: This software destroys anything it finds!!!