I tried to create a Starling account using my iPad. At the point where I was asked to enter my email address (immediately after DoB) I was unable to get the focus into that field. As a consequence I was unable to progress my application.
I switched to my iPhone (SE) and had no trouble at all. So I suspect this is an issue with the iPad version of the app.
It’s an iPad Mini 2 (32GB), on iOS 11.2.6 (15D100).