> Take a look at xmodmap

I did see that, but I thought it was going a bit out of the way for a whole 
keyboard layout! :-)

> "xbkcomp".

This looked promising, so I had a web trawl to discover how to go about 
creating an XKB keymap as input. 

Luckily in the process I happened to stumble across the very handy 
setxkbmap(1). A simple "setxkbmap dvorak" in .xsession solved the problem.

Thanks
David