Rachel, yup – it’s not really a big job at all and certainly something that even a newly qualified sparky can do with ease 🙂
Excuse the technical jargon but hopefully this will explain / help you understand a bit better what needs doing…
MCB (miniature circuit breaker) = protects the circuit / wiring from overload, pretty much the same sort of idea as the fuse in an extension lead but an MCB can be reset when the overload is resovled.
RCD (residual current device) = protects you from electric shock by detecting when current is going somewhere it shouldn’t for example running over the lawn mover cable or drilling through a cable.
RCBO (residual current breaker with over-current) = combines the functions of an MCB and RCD in one module
Your “old” consumer unit (fuse box) doesn’t have RCD protection and the law now requires that new circuits under most circumstances have it, this is why when the wet room was done the Hager board was added as it would have been easier and probably cheaper than trying to retrofit into the main house one.
In the case of the outside socket there are a few ways the electrician could go about it…
Fit another separate board similar to the small ones used in garages, sheds etc – these typically have space for 3 MCBs with one RCD protecting them. Is a bit overkill in your case but an option if they don’t want to touch the Hager board for whatever reason.
Fit an MCB in the Hager board and use an outside socket with its own built in RCD. Would provide RCD protection to whatever us plugged in but not to cable between the MCB and the socket. Depending on cable run / length this may not be an issue.
Fit an RCBO in the Hager board, provides RCD protection to both the cable and outside socket. RCBO’s are a bit more expensive than MCBs but on the flipside it means a “normal” and cheaper outside socket can be used instead of a more expensive RCD one.
Sorry its a bit long winded and quite a bit to take in but hope its helps.