Then it turns out the tool I was using was in fact the problem: it was incompatible with that website (something about only being able to do XML posts via SOAP for those of you who know what that means). Apparently what I had been trying to do to test it would have worked with any other website except for that one. So I spent hours and hours futzing with the data I was trying to put in (once I got something I thought would work), trying to make it exactly right (and thinking, surely I've just missed a little thing and that's why it's not working), and I was TOTALLY wasting my time.
I've accomplished almost nothing today. At least shadowdaddy and I went out and had a lovely lunch at Pu's Brasserie, the Most Unfortunately Named Restaurant In Central London (I even mention the possibility of "pus" brasserie until I looked at their URL), and lunch was really, really good, with a stunningly generous mixed appetizer starter in addition to the main (and no extra charge for rice).
I am now waiting for the videos of Hillary singing, "Soy un perdidador..." to come out on YouTube, but I think she just can't handle the concept.