Well I took Jack's advice about not trying to read both Treble and descant. A week at Whitby made quite a difference. At the start of the week if I was in a session and decided the tune was in Treble range I still couldn't link my ears and fingers. A week going to the Folk Orchestra and reading the harmony line not only improved my reading, but also playing by ear on the treble. Not quite mastered the octave skipping though (you sometimes need it on the descant too - though not as much).
Jack, next time we meet can you show me what a curved airway looks like.
In what way is it better? Which are easier to get the high notes on (if I could remember the fingering!)