So the app logged me out this morning, not sure why, so I logged back in via the email sent to me and this is when i noticed the Touch ID security feature to enter the app was switched off. I have always kept this switched on previously.
I turned Touch ID back on, closed the app and tested this again. When prompted to unlock Monzo using touch ID, i pressed cancel, and then log out. I then had to log back in via the email method, and again the touch ID had been switched off.
Therefore just to make the Monzo security team aware, this Touch ID security layer to unlock the app can be fairly easily bypassed: if someone had access to your phone unlocked (i realise this is unlikely, but can happen), and is able to obtain your relevant email address (which is fairly easy by searching the phone’s mail settings).