Hi Nick,
what you call a strong magnet is a "degausser" for hard disks this is expensive (much cheaper for tape technology).
I presume that this was a single disk and not part of a raid set (multiple disks all working together) if so go to your local hardware store and buy a cheap set of security sockets to match that of the screws in your disk, remove the outer plates and scratch the surface of the platter (the brown rusty looking bit), it will then take a real expert (ex is a has been, spurt is a drip under pressure

) to recover your data.
Your other option is to acquire a data eraser program from the net, there are many and some will even be to the strength that the military require (I erased a 20gb disk with full strength for fun and it took 2 days). It really depends on how paranoid you want to get (encase it in concrete and drop it in the deepest part of the ocean I would still not give you a written guarantee).
Or you could just start collecting disks in you attic and wait until the world has gone back into the dark ages and problem solved
Keith