I did the same last june.
The follwing is based on independant advice I recieved.
The legal situation is: the car has been sold as defective. (You can quote the sales of goods act)
You are well within your rights to refer back to the seller and demand that they rectify the problem or refund your money, or propose some other acceptable offer.
This is the line I took. I had to pester them alot
and they gave me a bit of a run around but in the end, to rectify the problem, they had to replace the red key which meant they had to replace all the locks, ECO and code box.
The garage I bought from is part of a large concern that did have ties to fiats.
Obviously if you bought privatly you may have difficulty with the previous owner. But any garage
should comply.
Also, you may have to be prepared to take into account any disount you recieved. However, argue your case. You did not now the fiat red key was so essential and you did not know it was 1200 pounds to replace. I assume.
Write to them immidiatly letting them know of the defect and your requirments. Keep all correspondance.
If you belong to an auto club, seek their advice.
Good Luck.