Thanks Alexis!
That seems to have done the trick: http://en.tervuren.treekee.com/wiki/Main_Page
The version of MediaWiki is 1.7.1 and the change is slightly different, as the line for the heading is a bit more complex, but the principle is identical.
In MonoBook.php, Before: <h1 class="firstHeading"><?php $this->data['displaytitle']!=""?$this->text('title'):$this->html('title') ?></h1>
After: <?php if($this->haveData('title') != wfMsgForContent('mainpage')){ ?> <h1 class="firstHeading"><?php $this->data['displaytitle']!=""?$this->text('title'):$this->html('title') ?></h1> <?php } ?>