Now... I was using an aging ADSL-USB modem, which I have just replaced with an ADSL-Ethernet modem. Wisdom says that an Ethernet connection is more robust, and has less CPU overhead than USB.
First impressions are that this has made a huge difference to SL. My connection indicator is now actively frisky whereas before it was pretty slugish. Packet loss, while still there, also looks a lot better.
Textures really are downloading much, much better now.

I've changed nothing else, so this might be a clue as to why some systems just seem to perform better than others in SL.