I find it embarrassing to ask such a simple question but it's a year since I did it and my memory has gone blank!!!!
I know how to use the Gradient Editor but I used to be able to make instant gradients based on the Foreground and Background colours near the bottom of the toollbar down the left side of the desktop.
I simply dragged down the image and a gradient appeared from top to bottom . . . . .
Now I can only make a gradient by selecting one of the presets or creating a new one in the Gradient Editor.
How did I do it originally?
It's working now!
I have no idea why it didn't do it before as I cannot see that I have done anything different.