Custom signup data doesn't include labels in emails, and has no theme functions

We used the signup module successfully in 4.7, with several fields added to theme_signup_user_form including some checkbox fields. E.g.

$form['signup_form_data']['Vegetarian'] = array('#type' => 'checkbox', '#title' => t('I require vegetarian food'));

In signup notification emails, the user's response for this field would show the field label so that the client knew which field had been checked.

After upgrading to 5.2, we copied our amended 4.7 theme_signup_user_form into template.php (as phptemplate_...). The fields appear correctly on the form but notification emails now just show 1 or 0 for the checkbox fields, with no accompanying labels.

Has the format of theme_signup_user_form entries changed in 5.2, or is there something else we need to change?

Thanks,

Dave