Graeme, thanks for the search idea.
My lsusb result is 12d1:1446. I wasn't sure whether I should use the 1446 or 1001, which is what most web site references quote.
I ran a few tests yesterday, gradually modifying my setup to see what happened. Most of my tests were based on the greenhughes.com articles (search the site for e169g). The first lines of the articles state that e169g should run straight out of the box since Ubuntu 8.10. Since I'm running 10.4 I'm disappointed. :(
It is possible (likely?) that I "corrupted" my laptop's capability to run out of the box, since I fiddled with the settings when the dongle first failed to work (through installing the simcard incorrectly). It is also possible that changes I made using 1001 instead of 1446 caused me problems, along with a spelling mistake copied from a different web site's "SUBSYSTEM" string.
I have now got to a stage where running usb_modeswitch with the codes 12d1 and 1446 no longer give me "No default device found". I now get "Prepare switching, accessing latest device" plus an error "sending huawei control message failed (error -1)". Which I conside a step forward. :)
It looks as if there is a problem recognising the dongle at all. I can't spend more time on it now but I'll report back in a few days when I have another go at it.