ECU light flashing when key is inserted, and not starting.
Exactly the same symptoms in fact as if a key with no transponder is used.
When it does this, turning off and on several dozen times (without trying to start) eventually clears it, and it stops flashing, and works again.
It does this rarely - once every several months.
To clarify - I mean to change the keys at the same time.
Doesn't the NATS identity of the car 'live' in the ECU?