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!!

...try lifting the floor
coverng: I'll bet there wil be evidence of where the hearth was.

BINGO! Found a slab of concrete about the width of the chimney breast
in the first floor room.

....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.

....not sure I follow this. The chimney breasts are all at the end of
the house on the end wall - two at the front and two at the rear (it's
kinda sideways on)

You could hang a length of cable or push a metal rod down the chimney
from the top and then use a cable/metal detector device to locate it
from
the outside or inside walls.

I'd have to be spiderman to attempt this. The house is 35 ft tall and
accessing the chimney involves scaffolding - lots of it.

And I thought this would be a fairly simple job - I can't believe it's
not as simple as drilling a few holes in the wall!!!!