Sue, I have recently suffered similar problem with damp stains in chimney breast in bedroom. After much hassle it was found the flournching was the problem. That is the mortar fillet that keys you chimney pots/flu etc. to the brickwork on top of chimney. We aslo took the precaution of re-pointing the whole chimney ie. belts and braces! Hope you find the cure. Tony DIY.
Anthony Dawson., September 2009