I'll admit this wasn't my idea, unfortunately I can't recall who suggested it... kudos to whoever it was!
Anyhow, they pointed out that the magnets you can extract from an old hard disc drive are so strong that you can stick them to the thin steel of an oil filter and enough of the field will penetrate to the other side that you could use it to help clean ferrous contaminants out of engine oil.
So I tried it last time I changed the oil.
6000 miles later I hacksawed the filter open and found that
a) a filter that's spent an hour draining still holds enough oil to make a hell of a mess.
b) the magnet had indeed collected a noticeable quantity of very fine black sludge
I think the oil flows from the outside to the inside, so I can't be sure that that wouldn't have been caught anyway... though I suspect it is fine enough to pass through the filter. Even if it would have been caught, at least it made the filter's job a bit easier.
So the magnet has gone back on the new filter.