Thanks for the advice. Actually, the most pertinent was making sure that I chose the correct Security Options setting.
Here you have a couple of options, each of which is progressively more secure than the previous. I decided to go for the 7-pass erase, which took about 5 hours to do!! Apparently it met the now obsolete US Department of Defense 5220-22M standard for securely erasing magnetic media by erasing the drive index files and writing over the data seven times. This hasn't been the standard since 2007 but if it was good enough for the Pentagon, then it's good enough for me!!
Lets hope the machine (sold on amazon) goes to a nice home. As far as 2007 MBPs are concerned, it's well looked after and sparsely used!!