I have located this, for anybody who needs to remove theirs, then read on:
Part number: 12567712
Price: £22.15 plus £1.80 off ebay from vauxhallpartswarehouse
DISCONNECT THE BATTERY, THIS IS A MUST.
To access the crank sensor, you need to remove the starter motor which is held on my two torx bolts, an 11mm socket will however undo them easily without damage to the bolt as they are not very tight.
You will need to raise the front end up of the car, you can just jack one side up if you like as this will give enough access to the underneith, use an axle stand for safety.
The top bolt is hard to access, but there is sufficient space to get an extension bar and socket onto the bolt from working in the engine bay, lay it on top of the starter motor and it will need to be say 2" longer than the starter motor, you must then go under the car and put the ratchet on and undo the bolt.
Before you undo the lower bolt, you need to remove a wire from the starter motor, there are a few, but you need to remove the large one in the middle as the loom is very neat and also very short, thus not allowing the motor to be withdrawn.
Unclip the oil sensor on the sump as the wires get in the way and now you can undo the second starter motor bolt, the starter motor still has one wire connected but you can rest it nearby and out of the way.
Now you will see the crankshaft / crank sensor, unclip the wire to it and use a 10mm socket to remove the bolt, and out it will wiggle.
Make sure you plug the connector into the new crank sensor after you fit it, and the wire to the starter motor, and also the connector back into the sump.
Re-connect the battery.
Quite an easy job really, just a little awkward.