A useful LINK relating to Supply and Demand, see below where to find the relevant section in the specification for Edexcel, AQA and OCR Economics:
Edexcel spec reference 1.2.6 
b) The use of supply and demand diagrams to depict excess supply and excess demand
c) The operation of market forces to eliminate excess demand and excess supply
AQA spec reference  Why excess demand and excess supply lead to changes in price.
OCR spec reference P 7  Supply and demand and the interaction of markets