In article , Rusty_Hinge
writes

Does anyone know what else we should try.

Sequestered iron, perhaps. Murphy do one called, IIRC, 'Sequestrene',
which also contains (again IIRC) magnesium and manganese. Manganese
deficience (often met with in Eastern England) is a contender.



Rusty I have to agree with an earlier answer which is that it does sound
suspiciously like box blight. Perhaps if the OP googled for pictures to
compare , that might help. If it is box blight I am not sure there's
anything they can do apart from grub it all up and burn the material.
Not sure if blight remains in the soil or is insect carried?

Over the past three years many people have lost hedges of box to this.

I don't think box i particularly averse to an alkaline soil if it was
then it would all be yellow.
--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk