Hopefully i can help..
Ok..if there is no signal from the key itself then that would mean either a battery fault in which you have replaced.."ensure it in fact the correct battery and is seated correctly" or the actual key itself is at fault or the rubber buttons on the key fob are worn in particualr if thats the only key you have the buttons are split etc/faulty circuit,microswitches the rubber buttons press on the fob.in which case another key would be required or repaired as such - there are many places on the web that can do a service like that so maybe something to look into - http://www.remotekeyrepairs.com/ - http://www.autotronics.co.uk/.. rather then the dealer if prefered to aquire another key,cut and programmed to the car...
There is programming procedure in the event the system malfunctions or battery replacement so maybe try that and see if that helps as below..
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I do hope this helps