nonymouse wrote:
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!)

You really need to find where the fireplace was; if you have four stacks
together then I'd be pretty sure the fireplace in each room will be
offset from the centre, so the 'middle' would NOT be the place to go!!

Is there really no evidence as to where the old fireplaces were?
Imperfections in the plaster? renewed section of skirting board over the
fireplace? Different sounds when tapping the front? Locate any metal
lintel using a detector? If you're really stuck, try lifting the floor
coverng: I'll bet there wil be evidence of where the hearth was. An
bear in mind if you can find where the opening is in room A, then in the
adjacent room B the opening will be on the other side of the chimney breast.

David