Sorry, I would delete this if I could. Found the solution. Still interested in the xmodmap/wayland issue.

I have used Xmodmap to remap the Windows key as the Caps Lock key. It is recognized as the Caps Lock, but it also pops up the Application Launcher from the panel. This is quite irritating, especially as it again pops up the launcher when I release the caps lock. Is there a way of switching this off?

I have been using this layout for several years, but never with Plasma before (I'm new to KDE). I don't want to have to change the caps lock key, partly because I'm so used to using it, partly because I have a minimal keyboard so not really any spare locations; I also have custom keycaps.

Not so importantly, does anyone know if xmodmap works with Wayland? I notice Plasma has a cutsomization section in the keyboard layout controls. I have not had time to explore this at all, so not even certain if it does the same thing, it just looked as if it might. Would this be an alternative to xmodmap?

Many thanks.