for those of you that think these arent worth bothering with,
i drained the gearbox oil, and found 1/2 a split pin stuck to the magnet
so i stripped the box down and found the other 1/2 securing a shaft in place that had worked loose and would have broken off and the shaft could have moved then a gear would have fallen off and BANG dead box.
so a small price tgo pay for a magnetic plug, as i wouldnt have known without it.