Overpayment - Lump vs Monthly - Would like to work out how much saving?

Hey all,

Pretty new to the forum so apologies if make any mistakes.  Ive been rattling my brain trying to figure this out and been going at it so long i have mentally over complicated things, so hoping someone knows the formula as i am feeling in my gut it is an easy maths problem.

I currently have a mortgage that has a fixed 1.64% interest rate.  Currently it has a £3,206.45 OVERPAYMENT allowance until September 30th 2022 i could clear.  I am currently clearing this buy paying £801 per month for another 4 payments.  I also have the means to pay the balance off in one lump sum payment.  Can anyone tell me how much interest total, i will be saying from paying it as a lump sum vs paying it off monthly in 4 more payments (ideally if there is a tool or a formula you can use even better, should i need to does this again in the future.

Thank you very much
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