I have now made the Fn-F7 and Fn-F8 keys useful.
I edited the .xbindkeysrc file in my Home folder again, adding the two lines:
m:0x0 + c:151
After logging out and in, this key now starts Firefox with my GMail page. The line in quotes could have been made to run thunderbird or kmail, etc.
As for Fn-F8, I decided to use it to switch desktops. Being as I make my panels hide, it's not always the quickest thing to switch desktops using the pager on the panel, so I opened KDE Control Centre/Configure Your Desktop > Computer Administration > Keyboard and Mouse > Global Keyboard Shortcuts. On the right hand panel, I selected for KDE Component: KWin. I scrolled down to "Switch to Next Desktop" and clicked to select the shortcut. Pressed Fn+F8, and "F28" appeared as the shortcut, which I saved. Whilst at it, I set Ctrl+Fn+F8 for "Switch to Previous Desktop". Applied the changes.
I now have useful functions on most of the keys.