The gge will have:
- a 3kW rotary 3-phase converter (probably needs a C or D rated 32A MCB for
starting),
- a 2kW 3-phase inverter (16A MCB),
- a regular ring (32A RCD),
- a couple of non-RCD sockets (16A MCB)
- and lights (6A MCB).
I'm keen to have the gge MCBs in the gge so I need a separate CU, I guess
the load is sufficiently large that it's worth having the 2 CUs in parallel
(?)

Ahh - this is a "serious" garage installation, then ;-) Reasonable to
reckon you could be pulling 7-8kW, being some 32A or more. For this
amount of "juice" you want to think of the feed to the garage as a
"submain": I'd want a 32A or 40A MCB in the CU, with the supply to the
garage in an Appropriately Rated cable; design calculations might allow
6mmsq, depending on length and circumstances of run, but you want to
keep voltage drop low on this submain and consider splashing out on
10mmsq. Depending on the physical run, it's just possible that PVC T&E
would do (if the gge is attached to the house), but SWA is prolly more
what you're looking at. The MCB in the main CU is only protecting the
cable to the gge CU; from there, protection is provided for each
individual circuit by the MCBs in each CU. You need to do the calcs on
earth loop impedance quite carefully to size this cable, especially if
you're going to use a Type C MCB in the main CU - since it takes longer
to trip, the permissible cable run is lower (to make sure the earth loop
impedance is low enough so it trips in sufficient time to protect the
cable thermally and limit the voltage while an L-to-E fault persists).
Basics of the calcs are well explained in the "standard" electrician's
bibles; but this is territory beyond the Which? Guide To Electrics ;-)

HTH - Stefek