When creating php pages in Dreamweaver MX (Win XP), I'm getting problems with odd characters appearing at the top of the page:

The're not visible in an IE6 browser window, but I can see them in Mozilla Firebird. Presumably they're something to do with the way DW creates pages, but I'm not sure what. The problem is they're screwing up my pages as they get sent before my session_start() code.
Anybody tell me (if/why) DW is adding these spurious characters to my files, and how I can get rid of them (They're not visible in notepad either)?
Thanks, njm