Well we just came back from holiday in France. My 2005 Freelander Td4 did well for over 3.000km until we finished our lunch on our way back. When I used the key fob, the doors unlocked. I put the key in the ignition all lights on the dash went on, the radio did it, but I could not start the engine. My windows and alarmlights etc. did it but it was like the immobiliser was not set off. We looked into the manual, and yes, my wife was right. The key fob had to be synchronised again with the car. You do that by quickly pushing 5 times on one of the buttons on your key fob and do that close to your car. We drove off, we stopped for diesel no problems, we drove 50 meters to park the car for a quick stop and when we wanted to leave, same story again. Now I'm back and haven't got the problem but changed the batteries in the key fob. Perhaps this is helpfull for your problem.
Good luck.