Make overpayments so why does monthly repayment barely change?

I have a repayment mortgage (tracker) and I've made overpayments where I've been able to. My mortgage lender automatically recalculates my monthly payments if I make an overpayment that's equivalent to more than three times my normal amount, so I've always overpaid by £1,000 to fall into that bracket.

When I first started to do this, my monthly repayments would go down by around £10. Even though the interest rate is still the same, if I overpay by £1,000 now, I'm only getting a reduction in the monthly repayment of around £3.

I have two questions - firstly, with such a tiny change in my monthly payments, is it really worth me overpaying £1,000 a time or would that £1,000 be better off going into my ISA and earning a little interest?

Secondly, why don't my overpayments reduce my monthly repayments at the same rate? I've heard that with a repayment mortgage you only pay off interest for the first few years, before paying off the capital, so is that the case here? I really don't understand how you can only pay interest to start with, when you continue to pay interest on the capital amount over the full term anyway - I need enlightening about how all this works!

Many thanks.