Changing the Header

Hi there, I was wondering if anyone could help me.

I am currently doing a bit of work on the company website (which is currently a mess). The agency that developed it originally made everything so complicated so that we had to keep going back to them to do the most simple edits.
Since coming on board I have said that I would take on some of the more straight forward Dev work as it would make more sense.

I need to add a new Page to the website and I have done so by setting up a new Page and using the Default Template. However the layout is slightly different to the rest of the site. I have tried getting round this by adding in the content directly into the page and then using scl_overrides.css to add in my styles. Apart from some caching that seems to be working OK, however the Header is different from the Default Header as my design has a Paragraph of text under the Title (a bit like a long tag line).

Is there any simple way of adding this to the Header I currently have? I have looked for Header.php in Appearances but I can’t see it. But I believe I have found the PHP file to the Header I would like to use which looks something like this: <?php get_template_part(‘templates/page’, ‘header’); ?>

Can i use a header from a different temple on the default template? If so how is this done? I have tried selecting that template from the drop down but them my HTML content disappears.

Thanks!