Hi Paul
The language string you would need to customise is 'forgotten' in core.php. Be aware that this will change the entire link for that string. If you have access to modify the php files you can add your line in the login/index_form.html - after the forgetpass div, around line 50:
<div class="form-input">
<input type="password" name="password" id="password" size="15" value="" <?php echo $autocomplete; ?> />
<input type="submit" id="loginbtn" value="<?php print_string("login") ?>" />
<div class="forgetpass"><a href="forgot_password.php"><?php print_string("forgotten") ?></a></div>
</div>
Alternatively and less problematic if you don't want to alter the code (because remember you'll need to manage this every time you upgrade!) is adding Instructions to the login page via the Manage Authentication Site Admin setting. This adds the text to the right of the login text boxes.
I hope this helps
Cheers
Teresa