David Hansen wrote:

However, if this is a spur disconnecting it at the other end would
be advisable. Alternatively, a switched fused connection unit ready
for some other gadget could be put on the spur.

In the end this is actually what I did. I removed the cable (which was
twin C+E) that was being "switched" completely. The mains supply to the
spur is still connected to the switched FCU, with the fuse removed for
paranoia's sake.

Thanks for your help, everyone.