I would have your chimney or flue checked ASAP. We had the same problem a few years ago and even after having the gas fire serviced it still left soot around the fire and on the marble above the fireplace.
We ended up chaning to a glass-fronted gas fire bought online from www.hotprice.co.uk
We went for this gas fire as we thought the glass front would stop the problem:
http://www.hotprice.co.uk/2118/2923/products/flavel-caress-he-high-efficiency-inset-gas-fire.html
It turns out that when we had the fitter remove the old fire there was a birds best in the chimney which meant that the fumes weren't able to go out at the top!
Very happy with the new gas fire though as it's far more efficienct than the old, open fronted gas fire we had.
Margie Stock
August 2009