For anyone interested - I still didn't find a "proper" solution to this. The best thing I have found was this:
https://play.google.com/store/apps/d...ocker&hl=en_GB
Basically, just keep it turned on so that the sidebar volume switch ONLY ever controls the speaker - not the mode the phone is in (i.e silent, vibrate, etc). I still don't understand why resetting my phone would cause this behaviour, as its bonkers to think you knock the volume button and it takes it out of silent mode! (that seems to be what was happening, as now I've not had it happen in 2 days - since installing that app)