Citroen Xsara Picasso hazard lights after locking keep flashing

Key fob has been slowly falling apart so I manually unlock but lock from the fob. This evening after locking the car the lights stopped their flash then 20 seconds later the interior rear light came on and hazard lights started flashing again then reset, stopped and interior light went out. Then a few seconds later the hazards started flashing again. I noticed the lock button on the key fob was stuck in so unstuck it and it came out the problem continued. Lights getting dimmer and then stopped so guessing battery is flat now. Not sure if the car will start in the morning so may be on the bus but at a loss as to what the problem is as if I get a new battery and problem not solved it won't last very long Help please