There are about 115,000 letterboxes in the UK. Or so says member number 2,744 of the Letter Box Study Group, a retired postman, Peter Willis. He has photographed 2,500 of them, and taking pictures of the remaining boxes has become "an addiction".
So committed is he to his mission that he was nominated for Dull Man of the Year in 2016.
Director Andy Oxley searched out other members of the Dull Men's Club, like Kevin Beresford, President of the Roundabout Appreciation Society, who's partial to a picturesque roundabout.
For others, it's milk bottles that float their boat (Steve Wheeler has a collection of 17,500 of them), or bricks (Neil Brittlebank's "longsuffering" wife Maureen has seen her lawn paved in them).
However, it seems unlikely that dullness will become a trend. As Leland Carlson, Assistant Vice-President of the Dull Men's Club explains, it's not a movement: "we like to stay put".