I have a black carpet with a grey twist. Black sofa with silver grey curtains. I have a black 'on wall' fire. What colour walls should I go for?

You have a lot of black going on mixed with gray accent. Gray is considered a neutral color so I would go with a much lighter variation of the gray you have in your carpet to break up the tone but still stay in the palette. Throw in pops of color to spice up the room.... With those colors you have lots of room to play with... Deep red pillows or wall art, gold, dark blues, etc.

Best Wishes!