I’ve just worked out how to get the job title and add it to the form, which I did by creating a hidden field in the render-form.php file on line 453:
<input type="hidden" name="title" value="<?php echo get_post_meta(the_title()); ?>">
Now, I still need to figure out what the name of the custom field is that stores the employer email address. The way WP Job Manager is coded is making it hard, but I’m getting used to it!