Electronic Anti-Theft Device Defective

Hi Guys,

My car is a 406S HDI 110, 2003 with 117000 miles. For a few weeks I have
been getting the above alarm message on attempting to start the engine.
Switching off and re inserting the key and the problem goes away and the
engine starts normally. But yesterday I tried this 4 times with no result,
the 5 th time I didn't get a message but the engine would not start, this was
as though the electronics had "locked out". I left the car locked up for 10
mins and it started normally on my return. Tried the spare key ( no
electronic fob ) and the message comes on with with EVERY attempt to start.
The engine turns over normally with every turn of the key and runs perfectly
if and when it starts. Should the spare key start the engine or is it just to
open the doors ?

Any ideas would be appreciated. Dave G