Landrover Discovery 2 alarm problem

I bought this vehicle 2yrs ago and within 4months the first thing I noticed was that as I was driving around a roundabout my fuel gauge registered empty and several lights on my dash board were illuminated. The next was when it failed to open when I operated the button on my key, the interior lights dimmed, the horn sounds and the headlight washers squirt water!!
Normally when it won't open I use the key in the lock to open the door which activates the alarm and then by turning on the ignition the alarm ceases, but 4 wks ago it would not stop sounding, wouln't start and the central locking/unlocking button on the car would not allow us out of the car, this continued for 45mins. I have had the fuse box under the bonnet changed, new door mechanism, as the drivers door button had stopped coming up when opening with the remote key buttons, and I still have the same problem. Please please can you help me