I brought the device indoors yesterday and it quickly responded - RH down to about 55% within an hour. So it (probably) isn't faulty. All I can think is that it is pretty damp around here (close to the sea, sun very low, damp grass nearby), so maybe it really is right at 100% where it is, and a more representative reading would be made in free air up a tall pole - which probably how it's supposed to be done, not screwed to the garage wall at 2m agl!