The above diff is for a manual car, and my view is that when you have fitted it, will it work?
If it doesn't the issue then, is was the installation ok, or was the diff defective prior? Fight it out between two parties, not an easy one.
I would rather have it fitted by a company who sourced the diff new, and fitted for me. Therefore any problems, and you only have to approach the one company, and they will be able to vouch for the fitting company.
£400 is ok, but not cheap enough to make the above decison easy, rather have peace of mind, and use a reputable company, with a bit of history behind them.