Living in the country, i have lots of friends with Land rovers who contantly have problems with the immobilisers!
I have found with 4 seperate vehicles (!!!!) that moving the car 50 yards or so down the road often gets over the problem!
I think there is a problem with the signal from the keyfob being jammed by either wireless internet / underground cables or the likes. I've never managed to find out exactly why it does it, but there are certain hotspots round here where it happens.
One of the chaps i know has sent his defender into land rover a few times to be plugged into their computers and no-one can give him an aswer why.
I have 2 mates (one with a disco, the other has a range rover), who have a 2 year old daugther. The wife (disco driver) goes to pick up the little one from child minders and has to park just down the road from where she usually does. Comes out to start the car and nothing. She calls her other half with the RR who comes to have a look, parks behind her, turns the car off to have a look at the disco. When he can't see whats wrong he decides to tow her home. Jumps in the RR and nothing! Wont even turn over. Push the disco down the road a bit, try to start it and away it goes! Same with the RR, push it down the road a bit and it starts.
Definately some weird electrical gremlins going on with the cars, which i can't explain why, but moving them a little seems to work!