SOOT FROM BOILER?

Since my Thorn Harcal 430 oilfuelled boiler was serviced in December, we are finding the top of the boiler and all worksurfaces in the utility room in which the boiler stands and the adjacent kitchen are covered in particles of soot. Our service engineer has fitted a new jet but this has not stopped the sooty deposits. Do you have any suggestions as to what might be causing the soot and how to eliminate it?