building a fireplace

after advice on installing a fire place please.i recently changed arund our kitchen and sitting room and now we have no fireplace in the sitting room.the cottage has 2 chimney's,1 at each end of the building,but the one in the sitting room only has the bricks that form the outside wall.i've seen it done on tv and sort of recall a concrete back being packed with sand etc.could someone tell me wht would be needed to rebuild the back and base.i already have a hearth and can easily build a brick fireplace surround as I am quite handy at the old d.i.y.any idea what sort of costs would be involved too