Chris Bacon wrote :

It's probably from where some furniture stood close to the wall,
restricting ventilation.

OK thanks for the quick reply.
This sounds likely as the wardrobe there was a freestanding one, but
had been screwed to th wall. I have tapped the plaster and it all
seems sound - that is a good sign I take it?

Anything that lets water drip or run down the wall won't help, but
that does not sound significant.

Ok, but there is not much I can do about this is there. Moving the
cable will move the problem (if there is any. But iill go up and see
if the tacking has loosened any mortar/bricks etc