Sorry to hear about the cambelt failure on your 156 JTD.
Unfortunately the days of ARUK paying out substantial cover for this sort of thing are long gone. The Fiat Group no longer have the funds and resources to meet the promised commitments of several years ago - the highly leveraged purchase of Chrysler saw to that. There's no harm in putting forward a case to ARUK, but I doubt whether you'll get anywhere. I would certainly be surprised if they accept the judgement of your independent specialsit in this incident, as others have indicated.
Your best bet is to cover your losses and get it repaired at a good specialist who will charge a realistic price for the job. The best quoted on here I think is AutoLusso at £850. I think the general average for the job at a specialist is around the £1k mark.
In the meantime, all JTD owners need to change the belts/waterpump before the recommended interval. I was going to correspond with ARUK with regard to the JTD waterpump issue (with reference to the sticky thread on this subject which I started), but given their complete lack of interest from any correspondence with anything to do with their products (such as the 2.2 JTS hesitancy issue, which they refuse to believe exits), that's probably a waste of time.
So enjoy the marketing of these products everyone, and enjoy the Alfa Romeos. Enjoy the sales speak, the espresso coffee in the dealership showroom, the smiles, the handshake, the handover of keys. Just bear in mind that, once you're out of the 3 year warranty, you're very much on your own.