After trying a new battery and the key still not working, I gave the dealer a quick call and asked if they had any further ideas. They didn't. Never being one to give up, I finally resorted to doing something I never do and turned to the manual for help.... First thing I read is that the C4 has a built in warning if it detects a low battery in the key! It'll make a noise, display an error message and light the 'Service' sign. The crucial bit is that after removing/changing the battery, you need to reinitiate the key by putting the key in the ignition and pressing the 'lock' button on the fob. After doing this the key works again! Somehow my key must have lost its 'link' to my car. Moral of story. Read the manual before calling dealer and ordering batteries of ebay! :blink: