Just an update-
On removing the rear internal heater cover , I found about 18mm of water sitting in the tray under the frost heater. I pushed a bit of wire into the drain pipe and the water drained away into the tray at the back, above the motor.
The end of the hose has a “sleeve” that looks a bit like a squashed/collapsed heat shrink tube that seems to seal itself. Is it possible that the collapsing in on itself is preventing the draining off of the water? It is about 1cm long, I was tempted to cut a tiny bit off the sleeve so that it has less internal surface area to hold itself closed but thought I would defer to your superior knowledge first.
Thanks in advance of a reply.