Text below the image in Header

Is it possible to have an image in the header but have the text appear below it, rather than just left or right?

You can use the Image field in Header, and below the image field just use the text field. Please check the screenshot below:

 

Thanks.

Your Quick Field 'Heading' (which I have used throughout my form) does not
allow me to add extra fields as you have described. Please elaborate - or
are you saying not to use this quick field and setup page headings in a
different way? Or is the Heading quick field only designed to be used once
...

Yes, if you're using the Heading field from the Quick Fields you can only set an image, set a heading text and set a sub-heading text as well. 

These are the only options you can set while using the Heading field.

What my colleague said was adding separated texts, so you will be able to add more text that the allowed by the Heading field. 

You may also use only the Text field, this will allow you to add images and the text that you want, you can format the text as you need too. 

Hope this helps.