When you transfer shares into your account, the book cost will be displayed as zero, unless you have provided us with a figure on your transfer form. The book cost will also be changed to zero if there has been a corporate action on your shareholding.
Follow these steps to edit the book cost of your shares:
Select the 'positions' tab in the side-bar and click 'add to workspace'
Click on the menu button next to 'market', then select 'book cost'
The book cost value will be displayed. You can adjust this by clicking on the value directly. See how it will affect the average price and P&L by clicking on the dropdown arrow to the left
When you're happy, click 'set book cost' or simply click elsewhere on the screen to confirm.