Simply deleting a file is insufficient with regard to putting the data beyond recovery. There are a number of apps available that can perform a total deletion of a data file – you can find links to many here:
https://www.lifewire.com/free-file-shredder-software-programs-2619149
These are not the only products available, and the products shown are not endorsed by the SU - this is simply a sign-posting.
Be very careful when downloading free software – make sure that you sweep it with anti-malware software before use. Also, be aware that there is no way of undeleting files that have been wiped using these products!
Hard copy data should be securely shredded after use – the SU Reception team can help with shredding.