It was a bad resistor.
Or rather -
When voltage was applied to the control pin of the magnet MOSFET, the resistor was fried immediately. This caused the MOSFET to be always on.
I've tried moving the magnet cable around and so far I've fried two MOSFET's and four resistors.
The two regular solenoid's on the same board works perfect.
I have checked the resistance which is fine, and the magnet seems to function both ways, i.e there's no polarity or diode in there. I think, unless it's been fried as well. I'll try putting one of my spare N1001 diodes on the magnet to see if that makes any difference - that's the one difference I can see at least.
I'm running out of spare MOSFET's here...