Given the amount of work it would take to rectify it (tracing the source
and
re-exposing the flex from under a fully-tiled wall!), and given the low
current drawn by the shaver socket, will I be able to sleep nights if I
just
nail the floorboards back down again...?

Personally, I wouldn't remove the cable. However, I'd be worried about its
short circuit performance, especially if it is on a 32A ring. I would trace
the other end and ensure that it came off a 3A FCU to protect the cable.

Christian.