Help with 'what is this' please

Bendy_House

So the 'wiring centre' - usually a rectangular white box where all the wires comes together - is somewhere else, perhaps near the boiler? Or even in the loft.

Swapping that valve head, then, will probably require an additional surface-mounted box fitted near where the wire goes through the wall, the existing wire cut and fed in there, and the new head's cable also going there - all joined with a 5-way terminal-strip.

(Either that or knocking a hole in that wall...)