My car is emitting smoke from the exhaust when the engine is revved after it has warmed up and the car has been driven for over half an hour. Is this normal?  If no any idea what could be causing it? 

Oil was recently overfilled at service by 10mm above max on dipstick - it was drained to below the max after being driven for 85 miles.  If the smoke is a problem could it be due to the oil overfill error? 

I would post a video of the smoke but not sure how to!

 

 

cant post video as file is too large

White or black smoke ? black would be oil burning off until its gone it will smoke ,white obviously coolant or dew burning.Get some redex once a month and cataclean every 6 months.Will keep the insdie of your engine clean...

Do they actually work?

is it work getting emissions tested - will that indicate a problem potentially caused by the oil being overfilled?

Yes they work why do you see so many Diesels and Petrol smoke, its due to the crap on the onside of the engines ,we all clean our cars outside and in but Engine hmm i bet allot don't. i have ben using Redex and Cataclean for years and it works without shadow of a doubt ! cleans fuel lines injectors carbon build up in engine etc ,biggest prob on diesels is the EGR valve carboned up because of oil fuel etc.Give it a go but yes never over fill oil it could also lead to other issues.

All cars produce visible smoke/water vapor in some conditions. At least when the weather is cold enough it should be visible. 

If the smoke isn't black and it doesn't smell sweet (I heard coolant vapor smells sweet) it's probably just a normal behaviour for your car.