|It think I've pretty well got it now.|
Firstly, I rarely use webmail and the message was indeed retained after the first download from the Win 7 machine. The change I saw was simply that it was no longer marked as unread. So that's fine, I was mistaken, it hadn't vanished from the server after all.
I'm assuming the weirdy whereby only the Vista machine cleared it from the server is some WLM magic and probably goes like this:
The Win7 machine was added later, so this was probably recorded by the server as an addition, the main (earlier setup) being from my Vista. It seems that the technology retains the webmail from the server until it has been downloaded from the main (Vista) machine unless it is deleted using webmail. I had no idea things worked like that and wonder if this is widespread or just a quirk of my UK server. As it happens it works out just the way I like it.
Ms Hulot's boat