[ic-page-header] storybook accessibility plug in complaining of two elements with role="banner"

[GCHQ-Developer-530]

Summary of the bug

On storybook examples of page header with top nav, the built in storybook accessibility plug in is failing due to two elements having a banner role and all roles should be unique.

🪜 How to reproduce

Tell us the steps to reproduce the problem:

1. Go to page-header sticky-with-content-and-footer example
2. Click on accessibility plug in in the drawer
3. See the error

📸 Screenshots or code

🧐 Expected behaviour

It should only have one banner.

Additional info

Weirdly page header doesn't have a banner role. This doesn't fail accessibility insights fast pass either so it's hard to tell if it's a weird quirk of storybook or not.