Last week's article on using accelerated mortgage payments to fund private second level education has sparked considerable interest from parents who have been struggling with this funding problem for some time. Two questions were raised by them which Family Money didn't address in that article: what happens to the savings timetable if interest rates go up and what impact would the higher payments have on the loan if the children didn't, in the end, go to the private school?

Last week we showed how an additional £84 a month (indexed at 5 per cent per annum), when added to a £663 a month mortgage could build up a sufficient surplus to pay for £28,000 worth of school fees over a 10-year period and pay off the remaining mortgage on schedule, in this case at the end of 17 years.

Our figures assumed a constant 7.5 per cent interest rate over the period, as opposed to the volatile rates that many homeowners experience in the course of a 20-year mortgage term. So what happens to this savings plan if interest rates were to go up by, say, 1 per cent, from 7.5 per cent to 8.5 per cent?

Although the basic mortgage payment will increase from £663 to £725 in the case of our example, "the credit you are building up in your mortgage account by paying the additional £84 plus every month is now worth 8.5 per cent to you and not 7.5 per cent," says our expert John Gilmartin of the financial advisers, Gunn Robinson O'Higgins. "Ironically, even though you are drawing down the same level of school fees, and paying a higher interest rate, you will actually pay off your mortgage a year earlier than if it had stayed at 7.5 per cent."

If your children do not end up going to the private school after all, any surplus payments you have paid will mean that your mortgage will be paid off much sooner. In the case we highlighted last week, it means the £75,000 outstanding mortgage is paid off in the year 2009 instead of 2014, four and a half years earlier. The savings in future mortgage payments amounts to over £40,000.