I'm using KDE Frameworks 5.36 and Plasma 5.8.7 on Kubuntu 16.04. I can't get compose keys to work with a US English keyboard.

I've tried lots of different compose key settings System Settings->Input Devices->Keyboard->Advanced->Position of Compose key. But no matter what I pick (caps lock, right win, left win, right alt or whatever) it simply doesn't work. Typing the compose key then e= for the Euro sign for example just types out 'e='.

Does anyone know what I have to do to actually enable compose keys in KDE?

Well the problem was fixed when I removed these 4 files from ~/.kde/share/config/ :
kcminputrc kglobalshortcutsrc khotkeysrc kinputrc

The current version of KDE puts these files under ~/.config. It's a bit irritating though that having the old, unused files there (which will always be the case if you don't manually clear out your ~/.kde/share/config/ when you upgrade from a previous version) causes bugs like this.