Think I have answered my own question. We updated the layout.css as follows:
background-image:url('logo_test.png'), linear-gradient(to right, #F6AB00, #F6AB00);
background-position:right center, left bottom 5%;
background-size:auto 70%, 100% 5%;
Although, at first, this displayed the yellow line, but hid the right-hand image, after I added logo_test.png as a baggage file, it worked perfectly, and added the yellow line under the two images in the header.
Please do. It's nice to be able to give something back to the forum after all the useful advice I have received from you guys.