When would there be a need for increasing the exit temperature, in order to aid dispersion of pollutants? i.e. what is the height OD of the top level of the cold air when additional energy is needed?

Changes to the exit temperature are not undertaken.  The stack has been designed at a height of 70m to ensure appropriate plume dispersion under all weather conditions modelled – i.e. 5 years of hourly sequential monitored meteorological data, and 2 years of site specific numerical weather prediction data supplied by the Met Office.