Do you have pingbacks enabled in the backoffice? See Blog settings -> Your Blog -> Feedback options -> Allow trackbacks.

The code you provided looks (is) OK. The validation page shows there is no pingback url generated.

I had them turned off because I was getting loads of spam. I just turned them back on and checked the validation page but it doesn't look like it's changed.

[url=http://validator.w3.org/check?uri=http%3A%2F%2Fwww.prettybloomers.com%2Findex.php&charset=%28detect+automatically%29&doctype=Inline&group=0]Validation page with trackbacks enabled[/url]

You have an error. This piece comes from the source of your page:

<link rel="pingback" href="<br />
<b>Notice</b>:  Undefined property:  Blog::$pingback_url in <b>/home/prettybl/public_html/inc/MODEL/dataobjects/_dataobject.class.php</b> on line <b>461</b><br />
" />

It does give you the validation error and since it doesn't work, you can comment out the line and wait for some one to come around who can determine what this error means and where it comes from. Once more; the code in your _main.php is correct, but something beyond my knowledge goes wrong.

Good luck

Update:
a search for Blog::$pingback_url in the forum revealed that you can safely delete the complete line:

<link rel="pingback" href="<?php $Blog->disp( 'pingback_url', 'raw' ) ?>" />

from the file _main.php since pingbacks are no longer managed in trackback.php but in a plugin. That explains the error you were receiving.

Good luck