User password won't let me login

So I've hit upon an odd snag. A few weeks back I was changing my master password for my admin account (to prevent my son using my iMac) and something went awry along the way and I now can't login. Oddly I'm no longer asked to input a password on the login screen so I can't do the 'reset password' trick. So my question is, how do I reset my master admin password whilst logged in?

I'm running Catalina 10.15.5 on a 27inch iMac late 2012