I had this with my series 3 turns out it was the Radiator cap needed replacing, the smoke on start-up is probably due to condensation or worn valve seals quite common on this type of vehicle. the first thing I would do is put an expansion tank on as the water from the radiator and block run into this when the vehicle reaches operating temperature. Also the white gunk you have on the oil cap may be due to condensation or a blocked breather, check the breather hoses and clean them, or take the vehicle on a long journey as short trips tend to cause this sort of condensation. I don't know what the weather has been like in your area but it's been quit damp here and I get "Mayo" in my rocker cover.
try the easiest solutions first and if all else fails take a more drastic approach!

quick update - it was the heater hose. since sorting it out the smoking has stopped almost altogether - slight puff on startup and then nothing. thanks again chaps.