HI again, (its more than a bit of plastic trim) firstly it won't be an allen key you need to undo the bolt you mention in the door it will be either a phillips screw or more likely a torx screw holding it, it's similar to an allen bolt but its just another way of the motor industry to deter you from taking it off yourself or you have go out and buy some more equipment, but you will need them again because lots of cars have these screws now, getting the interior trim off is easy if your careful but you will damage some no matter how careful you are because you will have no knowledge of how they come off so it will be trial and error, you can get a car interior trim removal kit off ebay and they're not that expensive, installing the central locking alarm you will need to remove some of the panels under the dash to fit the alarm module in a secure place, and removal of the windscreen trim for the interior sensors, front seat removal makes working under the dash much easier, removal of the trim in the boot, access under the bonnet bulk head to fit the siren, may have to drill the bulk head to gain access for supply cables, just stating the obvious but you will probably have to remove more than I've mentioned, you will probably need to get at the starter motor cables as a lot of alarms interrupt the power running to them, sounds horrendous but it can be done, you might be able to get a wiring diagram off ebay or try googling it, you will need a circuit tester also, a service manual won't tell you which wires to cut, the instructions for the alarm will but you have to find and trace them, not sure if Haynes has a manual for your car out yet but if it has it will explain how the panels come off and to colour code and identify cables but if you've never tried to follow a wiring diagram in one of these books I'm not going to spoil your fun, just make sure your mum isn't about in ear shot, I'm not going to throw any more water on the bonfire to dampen it down, just wanting to make sure you know whats involved, hope this helps and I've covered most of what you needed to know.