bmw 530d auto car alarm constantly on ? key fob fault?

hi
i have trouble with the alarm. yedsterday i locked the door using key in the door- CL worked ok....got my daughter...on way back i tried the remote unlock- didnt seem to work so put key in door....doors opened but car alarm+++ so tried to manually lock and unlock...to stop the alarm...no joy...tried remote fob.....but nothing happens ..not even sure the key fob is working now....and alarm doesnt stop...even if the beeping not going off the lights are still flashing...had to disconnect from the car battery terminals last night...today reconnectd the battery and same prob
have tried reprogramming the car key...but no difference...at a loss...bmw dealership tells me key fob battery is lifetime but they can replace the whole key at cost of 160...too much for me......any ideas????

annie