PLEASE HELP - Black artifacts on models after Unity update

I recently tested my game on a few new Android devices and saw that on some devices strange black atifacts and weird white streaks appear on/from some models.

After some source control play, I realised the issue seemed to be introduced after upgrading from Unity 2017.2.0 to 2018.1.6, as reverting to the commit before the upgrade fixed the problem, and the commit afterward the glitches occur.

Upgrading to Unity 2018.2 has had no effect on the issue and I dont know where to turn. The issue isnt caused by the toon shader on the models, as building out with a standard shader causes similar issues only the black areas are culled and rendered transparent instead.

Also as I didnt realise the issue was present for some time (didnt test on affected devices), I have done a LOT of work in the new version and reverting back to the old version of Unity isn't really an option now.

I have attached some screenshots of the issue, if anyone can help PLEASE PLEASE do. I need to get this issue resolved soon as we have an exhibition of the game coming up.