Single circuit CU

Such things exist - however they are taller, so not all CUs will have space.

Yup that would be fine. Just blank the spare way, or leave the MCB in place and label it as "spare" or similar.

If the branch was at the CU that would actually be acceptable anyway.

It would have to be a new enclosure as the existing one has an integral, non-modular switch and only space for one MCB.

That's because of the limited current demand from one socket outlet? I haven't actually checked where they branch because we only use one socket anyway and the other one has duct tape to avoid confusion.

ok, in which case all you can do is check the height of the available single module RCBOs, and if not swap the box.

Yup - you are in reality just talking about a spur - just one where its taken from origin. So the MCB will still provide fault protection for the single 2.5mm^2 T&E, and the limitation of a single socket will provide the overload protection. (overload protection does not *have* to be at the origin of the circuit (although often it is)).

In reality, (and depending a bit on the installation method for the cable) you would be hard pushed to do much damage with just a pair of sockets even if the branch was elsewhere.

Since most of these off-peak installations had surface mounted cables anyway, so surely RCD protection is not needed ?.

The man wants RCD protection, otherwise he wouldn't be changing it.

A nice shower/shed consumer unit sounds about right if it fits.

But socket outlets are now required to have RCD protection even if the cable is surface mounted. So the OP would have to install RCD sockets.

A photo of the enclose would be nice. I might know of an RCBO that will fit after looking.

But your suggestion of a double pole RCBO in an enclosure is fine. If space is tight you may struggle

It sounds as if the circuit is now supplying sockets and not just fixed heating equipment. So they would require RCD protection anyway.

Thanks for the advice. I'll put it at the bottom of my list of jobs that need doing. I'm only thinking of interfering with it because it is the only non-RCD protected circuit in the house.