it needs to be ventilated top and bottom. Each flue (there may be two). A single air brick will do. You might be able to have the top one opening into the loft and have the top capped. It is an advantage to have the bottom of the chimney dug out so it can ventilate with a duct under the floor. That will prevent draughts in the room and help ventilate the subfloor.
lola, we have done exactly that, blocked off chimney breast in dining room (30s semi), have a rather large air vent. We still have a mantel and a huge mirror above, but our 8 seater dining table is pushed up to the boarded fireplace. If you only want to block it, you can even do it yourselves. We didn't do it ourselves, but I painted the lot (mantel and the boarded up bit).
Hello, we would like to close up one of the chimney breasts in our downstairs 1930s open plan living room/ diner. It is in a semi-detached property on the party wall. I was wondering firstly who I need to get in to do this (simply a builder?) and secondly is there anything specific I need to consider? Will there need to be an air vent put in or anything like that? Thank you for your thoughts.