Andrew wrote:

Do RCBO's still consume power even when no user devices are
using power ?.

Yes

Assuming 6 in the consumer unit of a typical average house,
how much leccy is used per year ?.

The chip used on the PCB says max 10mA consumption, so that's a couple
of watts per RCBO, I doubt anything else on the PCB consumes much unless
it's in the act of tripping, so one or two quid each per year?