Except that it's too late to get the notes before an August 27 interview, unfortunately: the notes will take 30 days. Contacting your MP might help you get you some information before that time, if your MP is responsive and can get any information out of the visa office. And though I hate to disagree with my esteemed co-spreadsheet-updater, it sounds to me more like they are verifying your husband's parental rights if they are contacting the birth mother of your stepdaughter, so I would be sure to have all of that documentation organized, complete and be sure that your husband's familiar with it. It could also be that they will verify the validity of the relationship (and certainly it would be worthwhile bringing that documentation), but the mention of the birth mother seems to point more towards verifying that your husband has full legal custody for immigration purposes.