Can't return to Theory Test questions after touching "Book" on iPhone or smartphone

When you touch the "Book" button (highlighted on the image below) while viewing a practice Theory Test question:

You'll see the appropriate section of the Highway Code:

To return to the Theory Test question you need to touch the "X" in the upper right corner of the Highway Code window - sometimes (as in the above image) the "X" is not fully visible, in which case scroll the page down to see and then touch the "X" as shown below: