The message
from Nick contains these words:

My loft is fairly spacious and boarded out but surrounded with loads of
bin liners with clothes

Ideal breeding habitat for mice.

are mice known to inhabit loft spaces

Yep...we heard ours racing about the loft in hobnail boots at night.
Then honey started dripping through the bedroom ceiling, and it turned
out the mice had found a bees nest under the loft insulation.

or could this just be one mouse
that has become trapped in the loft for some reason?

IME, there is no such thing as "just one" mouse :-). Iirc, females can
produce a litter every 6 weeks all year round.

Janet.