Disc 2 key fob not working

My son has disco 2 and his key fob does not work. He has changed the battery in the fob and the red light comes on on the fob but the door does not unlock. If he opens it with the key the alarm goes off. Have looked at threads and other sites but seen no solution. The sun roof leaks of course and I understand this can sometimes be a problem. Grateful if anybody can offer a solution. Should have stuck to Defenders like me! Thanks in anticipation.

It's not unknown and some say it's a common problem with the receiver, located above the headlining between the rear sunroof and back door, suffering water damage.
If you can remobilise the engine then that indicates there's not a problem with the fob.