I originally thought the 'strip' of rubber was off the failed cambelt until I bought the new auxillary belt and found it was 1 groove wider than the old one!
I was supplied with an auxillary belt for my BX TD that was one groove narrower than the one it replaced. Horrible noisy thing - must replace it soon as its probably slipping slightly.
I just mention this as belts are defined by their length and grooves and it may be posible that you had the wrong one fitted. This may also have contributed to its failure.
I believe you can download a whole belt catalogue from the Quinton Hazell site.