Grateful acknowledgement is made to the following sources: 

• Figure 1a: The Open University
• Figure 2: The Open University
• Figure 3: The Open University
• Figure 4: The Open University
• Figure 5: The Open University
• Figure 6: The Open University

Every effort has been made to contact copyright holders. If any have been inadvertently overlooked the publishers will be pleased to make the necessary arrangements at the first opportunity.