325i won't start

I brought my E46 BMW 325i sport with a faulty ignition, the owner said that if you fiddle with the purple wire in the ignition switch while trying to start it it will fire up and work fine.
From this I assumed it was the connector in the ignition switch, which I replaced but made a ridiculous mistake; I thought I disconnected the battery the day before so preceded to cut the ignition with the battery still connected. After I cut one wire (the purple one) I disconnected the battery like I should have done in the first place.
All put back together, nothing happens, I've replaced the whole ignition switch on the left hand side of the steering column but now, when I turn the key no lights come up on the dash at all, it's like the car doesn't realise there is a key in the ignition. We've cleaned all the terminals on the switch and the ignition barrel but no luck.
However, if you touch the correct wires together in the ignition (like hot-wiring) the car ignition comes up on the dash and cranks but not start.
This tells me the engine and ecu are fine but the immobiliser has kicked in.
So two questions really, why isn't the car recognising the key is in the ignition and how to deactivate the immobiliser.

Thanks


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325i won't start