We found this on our living room ceiling at the end of last year. Was pure black and appeared flat with it's front legs pointing inwards. I can find nothing like it on the internet. The only thing similar is the crab spider from Australia but its not identical. It's not a great picture but I am not a spider fan and was taken at a distance on my phone! It didn't like being cornered with a glass and I remember it did seem quite agressive. It was about the size of a 50p piece, maybe bigger.
It's impossible to identify from that photo, but maybe Nuctenea umbratica is a possibility. They're often found close to houses in gardens, and occasionally one finds its way indoors. Some specimens are almost entirely black.