Apparently what I was trying to do on Saturday was right, so I obviously did something wrong along the way. Anyway, the answer is here: http://lists.infradead.org/pipermail/get_iplayer/2012-July/003163.html
Summary: Click "link to this video" to get the URL, and from it retrieve the pid, then: get_iplayer --pid p00symr7
(or get-iplayer for my distro-installed version).