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what if I made an overpayment of £3700 which is easily within my 10% annual allowance.
That's the point I was trying to make earlier.
If you started the year owing less than £37000 then you wouldn't be able to make a free overpayment of £3700 as that would be more than 10% of the balance at the start of the year.
(Some of the overpayment would be free, but not all of it.)
For most mortgage companies the 10% annual allowance is not 10% of the initial mortgage, but 10% of the balance at the start of calendar year.
I asked my mortgage provider how much they would charge to pay off the whole off my mortgage (a piddling £3700) and they quoted £36 so I then asked what if I made an overpayment of £3700 which is easily within my 10% annual allowance.
How we both laughed.