"But I thought that the high business confidence which has resulted in more goods and services being produced, would in turn increase AS. Why is this not the case?"
What you say is almost right, but it is not an upward shift in the AS (increase in supply). It is a movement along the AS curve (increase in quantity and prices).
If you draw your typical demand-supply curve, and then shift the demand up, then you will notice the new equilibrium is to the top-right of the original equilibrium. This represents an increase in the quantity traded of goods and services, due to a movement along the supply curve as a result of the shift in the demand curve. This makes sense because an increase in the demand curve means people are willing to pay more for their goods and services, so suppliers (on the same unchanged supply schedule) will increase output.
Here, I am trying to explain that an increase in the quantity traded does not necessarily imply an increase in supply. The supply-side has not changed, but there is now additional demand, so the suppliers can afford to produce more (not an increase in the supply - which would represent things like reduced costs of production).