Thanks, that's what i followed to set it up. I think the problem is with step 4 "In the box for Blog address (URL): change the address to the root directory's URL. Example: http://example.com"
This appears to require that the root be a directory (hence the trailing slash). However the site i'm developing requires that the root is a page which is to have the filename discussions.php (the index page being used for other something else). If i put http://example.com/discussions.php as the blog address setting it seems to take discussions.php as a directory name.
Is there a way round this other than URL rewriting?