Typing Czech characters on a UK keyboard

So, here I am using both KDE and Gnome on Debian systems and intermittently trying to acquire a smattering of Czech.

Czech uses the Roman alphabet, but has some accented characters.

I want to be able to type these characters from my GB layout 105 key PC keyboard under both Gnome and KDE. How can I do it?

(Back when I have a full answer for Gnome).