I'm trying to get my head around how keyboard buttons are understood by X. I've used xev to find keycodes for the row of once-useless buttons along the top of my screen, and defined those buttons as F13-20 with Xmodmap so I could actually make them work.

But something I'd like to do is find out how to actually create new buttons - so I could, for instance, instead of having the "Play" button defined as "F18", define it as "MyPlayButton"

Anyone know of any good guides that would help me get somewhere with this kind of topic? I've tried the man pages already, but they seem to require knowledge I don't have yet. . .

I see you are in the UK.
Do you not subscribe to "Linux Format" - I find it an excellent source of advice, and the DVD saves hours of download time (even with broadband).
They had an excellent article on keyboards a couple of months ago.
Their website www.linuxformat.co.uk is currently down, but I think they have old articles on it. I have a feeling that older articles are also available on the DVD.

Hope this helps.