This has just happened to me and the nice man from the RAC fixed by doing the following
1. Sit in drivers seat with all doors, bonnet, etc, closed.
2. turn ignition key so everything is ready to start but do not start engine.
3. sit in situ for 4 minutes without operating any electrical equipment.
4. turn engine off and open and close drivers door
5 turn ignition key so everything is ready to start but do not start engine.
6. sit in situ for 90 seconds without operating any electrical equipment.
7. enter your keycode and start engine
according to the RAC chap this resets the immobiliser, it solved my problem anyhow.
Peter A Johnstone, March 2008