I spoke with Apple Support who helped me find a workaround of copying the content out and into a new blank pages document but I still wanted to know how to edit it rather than dodge the issue
I worked this out:
Create a new document using the Invoice template
For the top line:
Click on the COMPANY NAME placeholder
Open the inspector Pane using the Format icon in the tool bar
The panel called Text should be displayed with three sub panels : Style, Layout and More
Select the Layout sub panel
At the bottom of this panel is a section called Borders and Rules
In this section you can choose the borders and rules for the currently selected section (the company name placeholder text)
Choose the line style, thickness, colour, position and offset to suit
For the Bottom line
Ensure the following menu item is ticked: Menu > Arrange > Section Layouts > Make Layout Objects Selectable
Select the horizontal line
Open the inspector panel using the Format icon in the tool bar.
In the inspector panel Select the Style Sub panel
Format the line as required
Lock the object style again by removing the tick from Menu > Arrange > Section Layouts > Make Layout Objects Selectable
Add more objects to the section layout by adding them to the page then choosing Menu > Arrange > Section Layouts > Move Object to Section Layout