Mark,
Now is the time to stay cool and get tough.
You probably have legal fees cover as part of your car or home insurance. Hook the details out and call for advice. Normally, the solicitor listens to your case, and will provide either advice - or will take on the case and write letters and take Fiat to court - whatever is appropriate etc.
If you don't have legal cover, then you should start by writing a letter to Fiat, copied to the dealer and recorded delivery to all of them. Explain in the letter - calmly - that your car has been off the road for however many days and that Fiat are not honouring their warranty to you. Make sure you include the phrase "time is of the essence" and give them as many days in which to fix the problem to your complete satisfaction as would add up to 14 if you include the days it has been off the road altogether so far.
The people in Fiat you should send the same letter to (list them all as copied actually on the letter - so they all know who you are writing to):
Giulio Salomone - Fiat Auto UK Managing Director (Slough)
Elena Bernardelli - Fiat Auto UK Marketing Director (Slough)
Rob Calver - Fiat Auto UK After Sales Director (Slough)
Richard Gadeselli - Chairman, Fiat UK Ltd: 10 Bruton Street, London, W1J 6PX - you may also want to call Richard on 020 7408 1912
Sergio Marchionne - Chairman and CEO, Fiat Auto Spa, Corso Agnelli 200, 10135 Turin - Italy
I don't have Rob Calvers number - but call Fiat and see if you can get it out of them. I think this guy is the one you need to call - see:
http://www.channel4.com/4car/news/news-story.jsp?news_id=13920
Also cc: the letter to Watchdog, TopGear, AutoCar etc.
If nothing happens and you can't get a solicitor to act on your insurance policy, then you may need to consider removing the car and taking it to another dealer. If you have to pay another dealer to do the work, then you should make a small claims claim online:
https://www.moneyclaim.gov.uk/csmco2/index.jsp
Good luck!