Update: I was able to contact an Appodeal engineer Nazar directly (thanks karpovpw for putting me in-touch) and he was able to solve my issue efficiently.
There was a bug in the Corona Appodeal plugin source code that wasn’t allowing banners to show (not sure if isolated to just the Corona plugin source or not). Nazar quickly identified this by replicating the bug and rectified the problem. He then pushed the plugin live (whilst I slept). It is now functioning without fail.
Presumably this was affecting anyone who was attempting to show banners using the latest Appodeal (legacy/release) plugin. So efficient rectification and identification of this bug was both valuable to all Corona users, Appodeals business interests and myself.
Agramonte: Yes debug mode/logging was on, using iOS I never received any useful output. Apart from ‘banner loaded successfully’ Thanks for your help on this also.