Thanks for that. I've skimmed over my log but can't see anything about a stack overflow but I went here:
https://moodkie.com/forum/viewtopic.php?f=13&t=1452 and applied their fix "It looks like this is related to a bug at Unity's end which they are yet to fix, despite them saying it would be available in Unity 2018.
Going to Window > Easy Save 3 > Settings and unchecking Auto Update References should stop it from happening in the meantime."
Doing this has stopped the problem and my game now builds and works fine so thanks again, I'd never have thought of that.
Edit: I've just found that having that turned off causes errors that kill builds unless you only turn it off right before a build so it's references are up to date. It updates on Play and Build so Play your game with it on, then turn it off and do a build. That worked for me. Annoying but it works.