Ok thank you for taking the time to reply.
I have just downloaded CCleaner for Mac with the intention of using the 'erase free space' facility. However, when I try to do this I get a message in the Details box that states 'Erase free space is not available for SSD's'. I have a fusion drive, 128gb of which is the SSD. Is there any way I can wipe the remaining (non SSD) HDD using CCleaner? If not then I have a program that is of no real use to me! Any advice would be gratefully received!