"Mike Barnard" wrote
| To remove it means removing the cable, which is buried in the wall
| under a flat plastic cover under the plaster. I really don't want to
| dig the wall out

You could cut and make safe (junction box) the cable above the ceiling, then
just tile over the remains in the bathroom wall.

Alternatively, leave it in situ. Someone in the future might want a heater
there.

| and ideally, if it's allowed in the regs, I'd like to put a socket
| up so I could put a cheap mini hifi system of some sort on a high
| shelf.
| It's obvious even without reading the regs that I can't (legally) put
| a standard three pin single socket up there. What, if anything, CAN I
| put there to supply a music machine?

Fused connection unit.

| (I know the damp atmosphere will take it's toll on the system,

its. And soapy fingermarks will do the Deutsche Grammophon vinyl no good.

| but the S.O. has to have her music when having her hour long soaks!

A radio feed into the bathroom is very useful for keeping track of time in
the morning rush too.

| We'll be using an el cheapo system, maybe 2nd hand. Disposable, in
| that sense.)

Rather than have mains appliance in the bathroom, why not put the music
system elsewhere in the house (main bedroom perhaps) and have an extension
speaker(s) in the bathroom. This is what I have done off my bedroom radio
with the speaker mounted out of reach of the bath. You can get
damp-resistant speakers but I didn't feel the need.

Owain