Thank you everyone who has helped me with this problem. Your advice has been really helpful. I tried to get advice from Fiat but they wanted £174 just to diagnose the problem and wouldn't give advice in person or over the phone. I went to Kwik Fit who kindly did a walk through of the exhaust for free (rear box is blown) and probed the manifold bolt holes and deduced that the bolts have indeed sheared off - they can't however do the work to remove them. I have therefore been back to the garage who spotted it and booked the car in to drill out the bolts, fit a new exhaust and make good. Nice teamwork everyone - thanks.