If anyone else should find themselves with an at times incessant chirping/squeaking noise appearing in the cabin, having never occurred before and appearing out of the blue, some time spent earlier today trying to trace it revealed it to be the rear seat centre armrest in the down position.
Armrest firmly stowed vertically in the seat back and the chirping/squeaking has gone!
Public service announcement finished
Armrest firmly stowed vertically in the seat back and the chirping/squeaking has gone!
Public service announcement finished