Hello Kev,
Pardon my ignorance, but are we talking petrol or diesel here?
If diesel it is likely to be the pre heater which tends to come on in cold weather and remain on for a while, and does emit diesel smoke and smell! after the engine is switched off. Not a problem as I see it, just a characteristic.
If it's petrol engined - worry!
Hope this helps.
Kind regards,
Gareth.