[ShereKhan]

I had to replace mine last month. I had to place the fob in the special compartment...it let me start the car. The display gave no indication that the battery was dead. I couldn't open the car keyless so had to use the physical key you flip out. The alarm then went off whilst I figured out how to open the compartment for the fob. It wouldn't start the car for a few minutes.

You can get the battery from Argos or Wilko pretty cheap easy to change yourself. With button battery you need to get the right way round. Also don't hold pressing top to bottom with fingers as you will reduce its life. Just use a tea towel or cloth so that you do not physically leave any marks/grease from hand..