Meanwhile I discovered these on eBay. They are framed, and therefore not flimsey like other clipons, they are fairly flat and not curved, which means they fit well to most prescription frames (at least those I have tried). And they are only 4 quid, including postage.
I was a little concerned about the apparent white frame, and whether that would be distracting, but the back of the frame (i.e. the direction you view from) is black, so it is not an issue.