The total height of the stack is 55m, this is the height from the ground level that it will be sitting on to the top of the stack.
Relevant to sea level the stack is +85m AOD (Above Ordance Datum), and ground level that is stands on is +30m AOD again showing it is 55m in height.
Surrounding ground level i.e. (Information Hub level) is at +50m AOD meaning that the stack is visible 35m above the surround ground level.
For comparison, the nearby Sheepcotes radio mast rises to 48m above the surrounding ground level. Note: AOD mean Above Ordance Datum, but in simple terms mean above sea level, so +30m in this case means 30m above sea level. In the UK the reference point of sea level is in Cornwall.