The alarm went off for about 30 seconds and then stopped. It didn't fade and die.
I connected the charger to the big black wire, making sure it wasn't in contact with anything else.
The correct key!!! How would I know if it's the correct key for the car? As for the spare key...typically, whether it's a worldwide thing or perculiar to the UK...when I bought the car (used) it only came with one.
No...I still can't gain access to the boot.
OK, I would check voltage on the black starter wire and if under 12 volts, definitely try to charge it that way. And for kicks throw new batteries in the keyfob.
My Crossfire flaked out once and would start then die as if the security system didn't recognize the key transponder. I was able to open the door, then hood and disconnect battery to reset everything and it worked fine after that. I also ordered a spare 2nd key for peace of mind as well.
If you can't get the hood open, one other idea that would be a reach but may work is this. (and full disclaimers, I have no idea if this will work ;-) but I would try it if desperate)
Wire up something like a headlamp bulb or a couple of them, and connect it to the 12v wire at the starter and to ground (earth). Let them run the battery down until dead, then charge it, maybe that would reset everything?
And I would keep fiddling with the key in the door, and look into getting another key made, and working transponder for emergencies.