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The only reason to overpay is if you fear you might lack willpower
I overpaid because it saved me money on the mortgage without needing to make money by using the money some other way. It's almost a sure thing rather than hoping your investments increase in value, which looking at recent history you'd be in trouble with.
This is probably truer now that interest rates are higher. Sure you can make more money with investments but your mortgage will cost you more.
I'm putting it in fixed rate accounts and will pay off a lump when remortgaging.
The only reason to overpay is if you fear you might lack willpower to stock to the plan, and end up spending it.
I wouldn't invest any money I'm going to need in only 3 years time.