Ageeing with Builder I think you'll find that if your fireplace is original and of '50s vintage the mechanism inside of and directly above the actual fireplace opening is an area and mechanism called the smoke shelf and damper (at least here in the U.S.) which is made of steel and easily holds quite a bit of soot. There's probably a "T" handle on the front of the fireplace that moves in and out adjusting the damper from closed to full open. Absent that, the adjustment handle could actually be inside of the fireplace opening near the top.
I'd expect that if you move that vigorously even more soot is going to fall, although the smoke shelf is fairly flat and may not empty into the fireplace.
Most chimney cleaners, even the better ones, simply don't clean that area very well, since their motivation is to remove soot within the chimney itself to help prevent fires (hence the clay liner which are actuall sewer pipe here) and it's rather difficult to reach effectively...