Everyone else makes 720-730mm high kitchen wall units but to tart my
kitchen up I need 850mm highnew doors. Anyone know of a firm that
supplies such units?

Non-bespoke units normally come in multiples of 180mm.

i.e.
drawerline base = 540 door + 180 drawer
full base = 720 door
full wall = 720 door
standard wall = 540 door.

You might find 900mm doors, but I suspect 850mm would have to be specially
made, unless the doors are so plain that you could just circular saw off the
top 50mm.

Christian.