I could fix directly into the breeze block on the back wall for main support
Yes, use long screws to screw right through into the blocks.
Endecotp - how did you fix the box section steels?
They sit in notches in the tops of the side battens.
You desk material sounds more flexible than what I've selected so maybe I won't need steels and just brackets....
Yes, probably. I actually assumed that the board had no strength and used bending equations to choose steel dimensions that would give a particular deflection at a particular load (I forget the actual numbers).