I was caught speeding just over 70 mph on a dual carriageway. I did not spot the reduced speed limit in respect of road works as it was late at night and no obvious work was taking place. Will I still be banned as it is usually a 70 mph zone? 

If a reduced speed limit is in place, it will be effective regardless of whether work is being carried out or not.  The punishment will depend upon the amount of the excess speed and the circumstances at the time of the offence so if you can prove that no other party was affected by the error, the Court may be more lenient than an offence at the same speed if there were workmen in the vicinity.