What size cable would you recommend using for the run from the CU to
the meter (MAIN) and the run from the meter (LOAD) to the junction box
(as these are effectively radial sections).

Minimum 6mm. Calculations might enable you to go 4mm, but this depends on
the length of the cables involved and the installation method.

This applies to all cables that hold the full circuit current, including the
T&E from the CU.

Christian.