Hi Blossom
Let's say you have a mortgage of €100,000 at 3% with 20 years remaining.
Your monthly repayment will be €554.
If you pay a lump-sum of €20,000, you will be charged interest on €80,000.
You will be given a choice of reducing the repayment to €443 or
Keeping the repayment the same and reducing the term to 15 years and 4 months.
If you just overpay without giving them any instructions, you will automatically reduce the term but their system might base it on the scheduled repayments, rather than the actual repayments.
If some event happens e.g. an interest rate rise or you give them an instruction, they will then recalculate the repayment and the term.
Brendan