Certain audio tracks not rendering
Managed to dive in to it and root out the problem. Despite all the audio playing in the preview there was actually a channel solo'd in the audio mixer. I only noticed this when I ran an update on Premiere, de-authorised and re-authorised it (because of another issue that happened) the audio in the preview started to play back correctly as per the track solo status.

Thanks for those who looked at the question and tried to help.