Thanks for all your help and sorry for asking some "newbie" style questions!
I don't know what I was doing with ptpcam at first because when I ran it again it connected to the camera, but the camera wouldn't respond to any commands. I then tried running the "trunk" build instead of the "reyalp-ptp-live" build I was running, and it worked! (I think this might be because version 2.3 of PTP is too new for ptpcam, whereas 2.1 was fine, but I'm not sure).
Anyway, I've just installed SVN and downloaded the chdkptp source code, so I'll compile that later and hopefully get it up and running.