The Residency Hurlingham stained beds

Booking.com refuses to refund my money from The residency hurlingham who made me sleep in a unit with a disgustingly stained bed and blamed it on me whereas I was literally there for 1 night and that stain is even old and growing mould and shows signs that it was a wet spot that dried out and even because brown. Their response time is very much poor and they allocate too much time to the place Of accommodation to temper and change things