I have accidentally admin rights on my Mac user account

Hello all.

I was trying to tweak my Mac user account this afternoon, (all I wanted to do was capitalise my name that appears in the menu bar), and upon completion I apparently didn't notice that I needed to re-tick the "Allow user to administer this computer' box...

In short, I've ended up being a 'Standard' user instead of an administrator. The trouble is, I can now no longer adjust any of the things that I need an admin user account for. (And, as I was the only admin user account on my Mac, it now has NO admin user at all.)

I have tried to follow a procedure I found to reactivate 'Setup Assistant', the first stage of which involved booting-up while holding down Command + S 'until text appears'. But all that appears is the log-in screen with the option to log in as one of the users. (Did something change with that major security update patch that was released at the back-end of last year?)

Can anyone please help me get my admin rights back?

Thanks in advance.

I'm running macOS 10.13.3 on a late-2010 MacBook Pro.