[MatthewH]

The "Kessy" (immobiliser) module lies underneath the front passenger seat. It's fairly common for these to become water damaged when the drains get clogged and water enters the car.

Sometimes the module will work correctly again when dried out, but often not. A replacement would need to be correctly coded to the car (although there are ways to repair your existing one depending on the extent of the damage). I would suggest removing the seat and check if the area has been in contact with water.