Reading through http://www.lincolnsonline.com/forum/showthread.php?t=59424 suggests that the crank sensor should have a low (circa 1-2ohm) resistance. Mine measures 390ohm whether or not it is placed on a bit piece of steel. Is this right?

I thought there were two or three types of crank sensor. Magnetic, VR and Halls? - Or are all these the same??

A VR sesnor is a coil of thin wire. It should be in the hundreds to thousands ohm range.
390 sounds about right.
If you hold a piece of steel next to the sensor and pull it away quickly, with a meter connected to the wires, it should pulse a reading.

Darren

set your voltmeter to read low voltages and then move the sensor back and forth towards a piece of steel.

You should see blips on the voltmeter and then you can rule out the sensor and wiring.