I booked a 3 night stay. On the day of check in, I received a message from the property demanding a 300 GBP deposit, a payment which they take *outside* of HotelsDotCom's system. You cannot check in unless you pay this deposit. I have been booking accommodation in London every week for the past 18 months; nobody else has ever asked for such a deposit. The booking fee is non-refundable according to their T&Cs; given that I had already paid more than the deposit amount for the booking, I reluctantly paid the deposit and checked in to the apartment.
The apartment itself is small and basic. A lot of noise from the street. Bedroom and bathroom are in a basement that smells of damp and soil. Broken wi-fi.
The following morning, I was awoken by loud, urgent, persistent banging on the front door, window and letterbox. I didn't answer the door. When I later left the apartment, I found a card had been posted through the letterbox from a bailiff company, to the effect of: "Contact us within 24 hours or we're going to perform a clearance". I took all of my belongings with me when I left that day, for fear of coming back in the evening to find that bailiffs had removed my belongings along with the rest of the contents of the flat.
I cancelled the remainder of my stay and received a refund for the two unused nights. However, despite all their assurances from that my 300 GBP would be refunded within 10 days, it still has not been refunded 6 weeks later.
AVOID AT ALL COSTS!