I've got R Reg 406 estate which I've had for abouttwo years now and am very happy with it. When I bought it one of the remote locking keys didn't work but I didn't bother as I had a key which did. Now this has ceased to work although when I press the buttons the red lights on the fob come on. The batteries in this fob are Ok I think having been replaced about 6 months ago. What could be wrong?