Thank you, Ken.
I have engaged Apple Support to resolve the issue. However, my Mac now has two hard drives ('Macintosh HD' and 'Macintosh HD - Data') and they have advised me to untick all check boxes under Software Updates under System Preferences (since the Security Update 2020-004 still wants to install) - even after going through a couple of 'restores' of the system.
I only purchased this Mac 18 months ago and already it is 'out of touch' with Apple. I've been buying Mac's for 13 years, but this will be my last.
I live with a significantly impairing disability and am unable to work, so I cannot afford to buy new software, etc, because the OS company releases an OS that no longer supports 32 bit apps. Hence, I cannot upgrade to Catalina, but have to stay on Mojave.
I no longer trust this Mac mini. Instead of the user accounts 'System', 'Admin' and 'Everyone', my Macintosh HD has the users 'system', 'wheel' and 'everyone'. Apple Support are looking into this. I no nothing of an account named 'wheel' as this is my only computer and has never had a user or other account named 'wheel'.
Severely let down and frustrated is how I feel about this whole update issue. If it's not meant for your OS version, then why does it show on your OS version as a required update??? (rhetorical question as only someone at Apple can answer that).
Thank you anyway for your reply. All the best to you.
Mark.