I would imagine it depends on the mortgage provider. With Nationwide your mortgage is essentially another account. So on my Nationwide app I have my current account, savings accounts and mortgage account. As such I have just sent up a monthly standing order to transfer money from my current account to my mortgage account (for the over payment).
The down side however is that when using the app you think you have money until you scroll to the bottom of the page and see a massive minus figure.
The down side however is that when using the app you think you have money until you scroll to the bottom of the page and see a massive minus figure.