Obviously they're not considering it seriously yet, not even with nearly 5,000 votes on the above page. Here's what I received earlier after another nudge in their direction:
Thank you for your message.
I am sorry to confirm we don't currently offer the mobile app for Windows Phones.
Nationwide have chosen to develop the app for iPhones and Android handsets as these have the highest volume of users and we want to ensure best value for our members. Too few people download apps for Blackberry and Windows 7 & 8 phones to make it worthwhile creating versions for these platforms at the moment.
Sorry for any inconvenience this may caused.
I would like to thank you for contacting us. If you need anything else please reply to this message.
It's pretty much a standard reply as I'm sure I've seen other people post the same (it even brackets Blackberry and WP7 and 8 in the response). In fact, the similar one I sent last December received a more optimistic reply. I'm tempted to close my accounts with them and take my business elsewhere.