I ran your numbers through the calculator as well
Regular monthly over payments of 100 euro would mean an interest saving of 23,990 and mortgage paid off 3 years 4 months earlier
Lump sum annual over payments of 1200 euro would mean an interest saving of 23,149 and mortgage paid off 3 years 3 months earlier
Are these calculations the same if I choose the reduce the future monthly repayments, as opposed to reducing the term?
If you decide you wish to continue paying 1484 each and every month until the mortgage has been cleared, it makes no difference if it comes off the future repayment amount or the term - the end date will be the same as you are paying the same amount off until the mortgage is zero. The calculations are based on this assumption.
If you choose to reduce the term, your monthly mortgage repayment remains the same and you simply finish paying the mortgage earlier - so something like 1384 a month at the moment. If you pay 1484 a month from now until the end of the mortgage, it will finish 3 years 4 months earlier. If you pay 1484 a month for 12 months and then stop, you will have overpaid by 1200 euro so likely to clear your mortgage repayments around 1.5 months earlier !
If you choose to reduce the future repayment amount, your mortgage will not finish earlier. It will finish in 30 years, but you will could end up paying less each month for periods of this. So lets say you decided to over pay by 100 euro for 12 months and then went back to regular payments, you would have 29 years left on the mortgage and would be paying roughly 1378 a month on it. If you done it for 5 years, monthly repayments would be around 1,351 euro and 10 years it would be around 1300 euro a month.
I know that does not sound like much of a monthly saving, but after 10 years you would have paid in 12,000 extra (100 euro * 12 months * 10 years) and will have save 20160 (84 euro (reduced payment)* 12 months * 20 years) *goes not factor in the time value of money*
The longer you do the over payments, the better the benefits obviously - 15 years would be 1215/month, 20 years around 1040/month and 25 years down to around 513 a month.
*Note* everything above assumes interest rates don't change etc and are based on using an online mortgage over payment calculator !