Thanks, but this didn't work either. Mozilla still just displayed the XML.
Maybe I should just admit defeat and say that this is a case of Internet Explorer perhaps not doing things properly, but at least doing them (unlike Netscape/Mozilla). In my opinion there's a lot to be said for getting things to work somehow rather than sticking to the rules and delivering something that just won't do what it's supposed to!
Thanks again for everyone's suggestions. Please post if you can think of anything else I might try.