You could use an online mortgage calculator, but it isn't that difficult to get a rough figure.
Interest rate is how much the bank charge per year for the loan.
Currently: 3.1% of 74,500 = 74500 * 0.031 = 2309.5
Will be: 2.75% of 74,500 = 74500 * 0.0275 = 2048.75
A saving of 260 in the first year roughly, about 20 a month. You won't see your repayment fall by quite that amount because you will be paying slightly more off the debt, but if you add up the reduction in repayment and the additional amount that you pay off the loan, it will come to about that.