Hello @chrisjdavies,
Thanks for writing to us
In case if you are using the X theme where you have set the page template as “Layout - Portfolio” to display the Portfolio item in that case you can not add or display any text or content. It would require custom development, I would suggest you contact a developer who can assist you with your concern or you can subscribe to One where customization questions are answered.
In case if are using the Pro theme I would suggest you create a custom portfolio archive page through the layout builder where you can add custom text and custom design your layout.
Hope it helps
Thanks for understanding