YOU. ARE. AMAZING! Thank you for your help with that, you and Rodrigo have been great.
Everything seems to be working as I'd hoped except for one thing and I've updated my pen below so you can replicate this.
If the mobile menu is opened, then the page is re-sized to desktop (so that the menu resets), then resized to mobile again, the mobile menu doesn't open again? But if I re-size it one more time it then works?! Not sure what's going on there!
https://codepen.io/Indy_dln/pen/WNgRRpm