Thanks for your suggestion Michael.
I have added the adjutment layer above my clip, and, on the Edit screen, I have dragged the top left white tab of that layer to the top right, creating an even graduation from start to end.
How do I now apply the sepia tint? I can go to the Colour screen, then Workspace>Gallery, and select the Sepia option, but I don’t know how to apply it to the adjustment layer, as on the Colour screen the adjustment layer isn’t visible, and on the Edit screen the sepia option is not accessible.