David WE Roberts wrote:
So please tell me there is something simple and obvious I have overlooked
which is easy to fix.

Identify and isolate the cable at the consumer unit end, then
neatly chuck a supply through an upstairs window and plug it
into a socket outlet. Make note to rip up floorboards next
time place is being redecorated and replace with fixed wiring.

I have sympathy. I got a belt from an upstairs landing light
that I was *sure* I'd isolated as I'd pulled the fuse, only
to discover that it had been wired to both the "upstairs" fuse
and the "downstairs" fuse. The previous owner had obviously
thought: the stairs have an upstairs light and a downstairs
light, so of course it has to be wired to both fuses, doesn't it?

JGH