Quick update. Managed to install the CU. Couldn’t cut the power without putting the whole row of houses in the dark, but just capped the live wire off and was extra careful, even wearing rubber sandals while standing on a wooden chair ????. There was no sign of grounding anywhere, so I connected the N directly to the RCBO. It took a while to figure out why one group kept tripping the RCBO, but some genius had wired the porch light with the switched live wire coming from inside and the N going directly to the meter, bypassing the CU. All good now.
ps, the aircon is a 12k BTU inverter in a tiny bedroom, so should be fine on a dedicated 10A group.