Smoke from engine?

I have a 2006 Ford Fiesta 1.4TDCI 124,000 miles, regularily serviced.
Just started to smoke from the oil filler cap and under the air filter housing, seems to be like an engine breather problem. There has been no other drop in performance or speed just wiffs of smoke. correct grade of oil is being used, engine is not getting over hot. any help ?????