I’ve just come across this app, Sprive.
It’s described on the Play Store as -
Sprive is a mortgage app that helps you pay off your mortgage early
Sprive is the mortgage management app for homeowners looking to pay off their mortgage faster.
Sprive is completely free and only takes a few minutes to set up.
Has anyone come across before it? What are your thoughts? How does it work? 
Looks like they are an e-money institution rather than a bank or building society. That would rule it out for me.
Also, I spotted this at the bottom of their page:
Savings illustrated are based on average £92 paid monthly as an overpayment on a repayment mortgage with a balance of £250,000 at 5% interest for 30 years.
I suspect you could do that with your existing mortgage with most providers, without involving a 3rd party. Set up a standing order for whatever they will allow as over-payment.
You can pay it directly, but check the maximum you can pay before their early repayment charge kicks in. For instance, with Vrgin I can overpay by 10% of the outstanding balance each year.
Even if the early repayment charge kicks in (5% for Vrgin) it may still be worth doing.