I would put a vent in the wall where the fireplace opening used to be,
and then put a ventilated cap on the chimney pot.

Don't faff around putting air bricks in the loft.

You either fit one air brick per flue, or you knock a gap between the flues,
so that one air brick will cover both. It is better to ventilate to the
outside than to the inside of the house, as it looks neater and is more
energy efficient.

Thanks Christian. Ideally I would like to fit one air brick per flue
but finding exactly where the flues run when the chimney breast is 5ft
across isn't easy.

Thanks to all who posted. Have decided to fit one on the ground floor
and one on the first floor fairly central on the chimney breast (and
hope for the best!)