Done. 60 years of a damp musty-smelling under-stair area, 30 mins of work, 2 days to dry out and stop smelling. A completely passive solution. Sad that generations of people had to suffer because nobody could be asked… We do live in strange times!
The vent simply connects the crawl space under the staircase with the lounge / living room thereby allowing a through-draft under the stairs (air comes in the slatted door to the crawl space area and now comes out the new vent into the lounge). You can feel the passive airflow if you put your hand in front of the vent.