Thanks for that, so to clarify I have 2 RCBO's (Sockets and Garage Sockets), identified as they have the black switches towards the top? Seems strange that despite being a mix or a RCD and a MCB it uses only 1 slot yet a singular RCD takes up 2? Have I misunderstood something here?
RCDs in the consumer unit usually cover multiple circuits, are usually rated at > 63A, and break both live and neutral. As Winston says, they are usually 'double width' taking up 2 slots.
You are right the RCBOs have the little black test buttons; they are physically larger than MCBs despite only taking one slot, but typically they only isolate the live.