Well, I went to Screwfix this morning and bought one of these door stops. When you take it out of its plastic bag, it does indeed seem to have an incredibly strong magnet. So this afternoon I decided to fit it to the forecabin door. More than an hour later, I came to the conclusion that this incredibly strong magnet is only incredibly strong when it's perfectly aligned. A fraction of a millimetre out of alignment, and it's much less strong. Take care when fitting!