The code card used to be issued with the car and allowed you to start up if the key code didn't work the instructions for how to use the 4 digit no. should be in the handbook.
The cards are not now issued at the request of the insurers but you should be able to obtain a code from your Fiat Dealer...yes I know thats a long shot but it can be done if they have the will...You will have to provide them with Proof that the car is yours, its chassis and original registration No.
The dealer should then fax fiat uk with the details who will then supply the code...In my case the number had been noted by the supplying dealer on the PDI documents....is that really any more secure than issuing the card to the rightful owner who may just need it some dark snowy night 100 miles from home???
I suggest you get a note of the code and keep it in a safe place just in case...