patch tool is in a hidden toolbar and when I try to drag it into the main toolbar, it is not highlighted and so not working, it is not an image or layer issue, but the tool itself!

Hi, the drag has to happen inside the Customize Toolbar dialog, not on the main toolbar itself:

1. Choose Edit > Toolbar to open the Customize Toolbar dialog.

2. In that dialog, the Patch tool will be in the Extra Tools column on the right. Drag it from there into the Toolbar column on the left.

3. Click Done.

If that still doesn't work, the quickest reset is Edit > Toolbar > Restore Defaults, then check the Patch tool — it sits in the healing tools group (with the Spot Healing Brush). You can also reset the workspace via Window > Workspace > Reset Essentials.

Official Adobe doc: https://helpx.adobe.com/photoshop/kb/bringing-back-tools.html

Hope that gets it back for you!