[Bug]: Header spills into main content on smaller screens

[sarahmwong]

Overview

Header spills out on to main content when a screen width narrower than the header is used.

Steps to Reproduce

Run example app

Reduce screen size/view in mobile mode

Expected Behavior

Expected behaviour to widen the blue bar so the full header is still intact on the blue background when this happens

Additional Information

Introduced when new gov.uk frontend header was added