2003 Rover 25 exhaust smokes a little

Smells problem
2003 Rover 25 4 cyl Front Wheel Drive Manual 66k miles

hi I receintly purchused a rover 25. The car was smoking quite badly so I had my local garage fit a new head gasket. The problem now is car isnt smokey at all on startup but smokes when warmed up its not that much but its more than it should be. How long does it take for exhaust to clean itself could this be the problem any help would be greatly appreceited thanks paul