Dear Fanis,
The problem seems to be quite simple. The installer has not removed the loose black "pre-installation" bolts that are supplied purely to keep the diff in 1 piece. These are supposed to be removed and replaced by the separate silver, pre-loctited bolts which are packed separately, and torqued to 37Nm.
So the installer has added some correct bolts, but has not removed the black ones. So there was one bolt that fell out of the Quaife, because it had no loctite, and was almost certainly not torqued down. The bolt did not break, but merely fell out of the housing.
All of the stock that we had at that time had a clear warning lable on the carton, and also the reason for the provision of these bolts was adequately described in the instructions, which were emailed to you prior to your purchase.
So this is not covered under warranty, as it is an installation error.
Regards, Kevin