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..

http://www.bankrate.com/brm/calculators/mortgages.asp

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!