Without seeing the lie of the land,order of works.
Excavate footings, and concrete, include starter bars set in concrete.
Lay 225 hollow blocks,starter bar set in hollows.
Extend bars to full height, lay blocks to 1.8m
Fill all hollows, solid with concrete.
Footing size allow for 1m deep 600 wide.
Lay good drainage behind wall , with clean hardcore.
Allow for ties on front face,to allow for a facing wall, ie brick/stone.
Approx £2630 not including a facing stone.
I would go over the top for the footings as, there will be quite a bit of weight behind wall.
These prices could vary as a lot depends on access.