From the 1st of October 2015, anyone caught smoking in their vehicle (whether they are a passenger or driver) whilst having under 18's present will receive a £50 fine. It doesn't matter whether the windows are open or not, or the engine is on or off the fine will still apply. Apparently the only exception is if the driver is in a convertible car with the roof off. It is understood that if you are alone in the vehicle then you can still smoke and not receive a fine but there could be punishments if as a result you fail to drive with due care and attention.
This law will only apply in England and Wales though Scotland are likely to adopt a similar approach in the near future.
I've always wondered why they have never banned smoking whilst driving as surely it's a distraction as well as being detrimental to one's health.