Citroen xsara wont start, clicking noise and immobiliser light on?

Citroen Xsara 2004. My problem started a couple weeks back when the battery warning light came on and 3 beeps. Sometimes it wouldnt come on though. One night the car wouldnt start first time and when it did start the engine management light came on. I drove home but it wouldnt start in the morning and still wont start. It would turn over but eventually the immobiliser light would come on.

I thought it might be a dead battery so I charged it fully and put it back in but when I tried to start it, I got a fast clicking sound coming from near the fuse box in the engine which stayed on even after I took the key out and I could only get it to stop by disconnecting the battery.

Can anyone help?