The page is tight up against the top of the browser window, so there is no apparent gap for a "missing" element in the page.
I have just made a couple of changes to the settings and have a result!
Originally in Designer Pro I had the banner in the "Header Background" slot and marked as 'Visible'. I had nothing down for the Logo, which I had removed by clicking the red button with the white cross as the 128 x 128 standard logo was obscuring part of the header.
What I did was to treat the "Header Background" as 'transparent' (thus removing the jpg I had originally used and which appeared in the themes pictures) and then replaced the 128 x 128 logo with my banner which is 750 x 108.
In other words my whole banner picture becomes the logo, but with larger dimensions than the logo supplied with each template.
The result is that my test page in the browser shows with my banner picture.
All is well with the world!
Thanks for your suggestions. I hope the solution will benefit others who come across this problem in the future.