Is it possible to edit the testimonials page - I want to add a nice image header to the top of the page
and also the form should be at the top of the page with the testimonials under that.
You could add an HTML Block above the Main Content of the Testimonials Block content. Design > Blocks > Testimonials tab.
Place your header image in the HTML Block and make the block active.
This way you do not make any changes to the code so you get worry-free upgrades.