How do I fix my smoking chimney?

Just had it lined, old chimney partly shared with neighbour.(mid terrace) I opened the fireplace up as it was only used for a gas stove before I bought it. Now it is smoking unbelievably, builder finished it yesterday and came back and put a cowl on top today(spins around on top), but it makes not difference. Sufficating amount of smoke blowing into the room. Help please.... Neighbours who I share with say they have a bad smell of soot from theirs. Neigbours a few doors lower say no problem. Very low lying village.