Whether or not the incorrect fluid level had caused a deeper issue, I don't know, but said gearbox gave up the ghost.
Replacement gearbox sourced, engine out and cleaned up engine complete with replacement gearbox fitted ready to go back in!
Engine removal wasn't as daunting as I thought and in fairness I have a four post ramp which made the job a whole lot easier. Hope to get it back in this week and then road test as soon as possible.