As to why it will have broken, chances are its just the belt that was replaced, and it has snapped/jumped off because the tensioner has siezed. I don't think you'll have much luck getting the original garage to rebuild the engine, if it was 13 months ago, especially as it wasn't you that paid them to do the work and you don't have a reciept for it, so nothing to state they gave a guarantee of any sort, other than under the Sale of Goods act, but you are probably out of the scope of the law anyway here, as so much time/milage has passed.
Did you buy the car from a private seller or trader?
Also, are the people who did the cambelt an Alfa garage? Or just a local garage?
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