Absolute nightmare to deal with. Still waiting on our deposit over a month after our accounts were closed and everything was supposedly sorted. Make sure you attend the inventory session when you move out and ask for a copy of the final report - they are trying to charge us for something that they claimed is broken but was not on the inventory. When I mentioned that myself and my flatmate had been present at the time of the inspection and that we were personally shown on the detailed inventory list that nothing was wrong or broken with the flat they suddenly went a bit quiet!
Also, as a warning to female tenants in particular, be aware that any requested repairs will likely be done unannounced, with no warning whatsoever. I awoke one morning around 8:00 to find a man in my hallway claiming he'd let himself in and was there to fix the fire alarm (which, coincidentally we had reported over week prior to him showing up - how's that for health and safety!). Um, hello. Thank you for the notification! Scary and made the entire process of requesting a repair rather unappealing (that and the fact that it usually took about six calls for a repair to actually happen). Unauthorised and unannounced entries occurred twice again after this incident, which I would have possibly understood if they were a day or two after making a repair request, but not after waiting weeks of hearing nothing!
If renting a furnished accommodation, know what you are entitled to as a tenant, they will pressure you to sign a lease ASAP citing the competitive nature of the London lettings market. We unfortunately signed off on accepting that they wouldn't provide some pretty major furnishings stupidly thinking we could get them secondhand - they should be providing tables, crockery, curtains, etc for you, especially as they charge premium rent!
As an expat living permanently in London I'm frequently asked by friends who are moving over about letting agents. I specifically tell people to avoid Black Katz as they have proven themselves to be unprofessional, disorganised and completely unable to comprehend the concept of customer service. Considering the extraordinate "administration" fees and inflated rents they charge, this is truly disappointing. Obviously, service may depend on the branch but judging from many of these reviews, poor customer service and sneaky business practices seem to be a bit of a general trend.