you've got 4 times as much interest, but not 4 times the capital.
if you borrowed 200k (the exact amount doesn't matter, as the dates will be the same regardless) on your 5.85%, over 25 years the repayments would have been 1270 pcm. of your first payment, ~£300 is capital, the other 970 is interest. all good.
now, if rates the whole time were 1.35%, the minimum would be £785 pcm. your first payment is £560 capital, and £225 interest. the blend of these changes over time, so your last payment is nearly 100% capital.
now, if you choose to overpay by £485, bringing your total payment to 1270, your first payment will be £1045 capital and still £225 interest. Your loan will be paid off in 14 years, 5 months, provided rates remain the same. 10 years off your mortgage is a bloody good thing, but not as good as 20!
this tool is very useful for doing this sort of calculation..
even if you paid no interest at all, it'd still take 13 years 2 months to pay off the mortgage at 1270 per month!