If a genuine Nationwide customer's account had a problem, Nationwide would contact them by phone.
I know this because I was paying for a laptop last week and the payment wouldn't go through ..Nationwides automated system decided it was possibly fraud because I'd bought a small item from the same company the week before
It wasn't a problem - I just needed to verify that it was indeed me making the purchase. Couldn't really be cross with their caution.
They do send emails regarding confirmation of new savings account applications, new products etc, but they always have the full postcode top and bottom, and they address one by name.
I'd never click on a link in any financial institutions emails, even if I was virtually convinced it was genuine.