SOMEONE putting the downpayment on their first house seldom thinks about the total cost of their purchase, usually more than double the cost of the loan.

Their focus is on meeting each monthly repayment as it comes; their hope is that no crisis will occur to strain their resources and ability to pay. With typical mortgages now in the region of £60,000 monthly repayments can easily reach £500 or more when insurances are included.

First National Building Society has just brought out a flexible mortgage product, Mortgage Master, which is worth careful consideration, whether you are a First National customer or not, because of its potential to pay off your loan sooner, thus saving thousands of pounds in future interest payments.

Central to this product is the ability to adjust the way the loan is repaid to suit your financial position and needs at any given time. By making a series of early stepped-up payments, or lump-sum payments, you can opt to pay off the mortgage early if you so choose; to take payment breaks or to use the built-up surplus in your account, which First National describes as an "overpaid balance", to make cash withdrawals.

This is one of the first "flexi-mortgages" on the market that permits a combination of these feat tires, rather than offering them as separate products. (Bank of Ireland and National Irish Bank were the first lenders to bring out separate, flexible mortgage products.)

There are no additional charges or fees for Mortgage Master and no premium charged on the interest rate. The product is as suitable for existing customers as it is for new ones, though First National admits it expects second-time borrowers will better appreciate the significance of paying off the mortgage more quickly than first-time buyers.

The first Mortgage Master option is early repayment. The customer increases the regular monthly payments or lodges a lump sum (this can be a substantial sum, say, an inheritance or proceeds of the sale of another property or shares, or a more modest amount a quarterly bonus or commission payment).

The overpayment is deducted from your outstanding capital (First National makes these capital deductions on a monthly basis) and the future interest you would have paid on that capital is avoided.

The more overpayments you make the faster your loan is paid off, saving years from the loan period and thousands of pounds in extra interest. The higher the interest rate, the greater the long-term savings, something that should be noted by fixed-rate customers.

First National gives the following example (see chart) of a first time borrower with a 25-year mortgage worth £75,000. This person decided to increase his repayments by 3 per cent per annum since that was his annual salary rise.

Assuming that interest rates remained at 7.2 per cent over the term of the loan, this borrower would have saved £26,891 in interest and knocked eight years and seven months off his loan period. He can stop his overpayments at any time without penalty.

Option two allows anyone who has built up an overpayment to take short or longer-term payment breaks. Short-term breaks can ideally be taken at Christmas or during the summer holiday period, the company suggests. It describes the breaks as having been "pre-paid" by your over-payment in the previous months. (You must have made sufficient overpayments to fund the payment breaks.) Longer-term breaks will appeal to borrowers who take career breaks, are changing jobs or are on maternity leave, etc.

For example, someone on a £50,000 loan over 25 years steps up his payments by £50 a month for the first seven years. He then takes a six-month payment break during year eight and resumes standard repayments thereafter.

The monthly amount he normally would have paid is deducted each month from the capital he has built up. Even with these breaks this customer still saves £8,833 and has reduced the term of his loan by two and a half years.

Option three is a variation on the above which involves paying more into your mortgage account, and then simply reducing the size of your payments, either back to the pre-escalation amount or lower for a period of time that suits you - say, the years when school fees need paying as well as the mortgage and other commitments. What you may not do, in setting your lower payments, is exceed the extra amount you already paid in.

The fourth option is a series of staged withdrawals. The Mortgage Master allows you to withdraw all your built-up overpayments either in staged amounts or all of it at once.

The company gives the example of a person with a £75,000 mortgage who has lodged a £3,000 lump sum into the loan and increased monthly payments by £100 per month. She withdrew £5,000 each year at years five, nine and 13 as money towards a replacement car. On this system, despite the substantial withdrawals, she still saves £21,799 in interest and five and a quarter years off the term of her 25-year mortgage.

First National has used 25-year lending terms in its examples, and while the impact of escalating payments does not took quite as dramatic over a 20-year term, the company insists that many new mortgages are negotiated over 25 rather than 20-year terms.

Other lenders offer a variation on this product - Bank of Ireland has had flexible payment mortgages, index-linked accelerated mortgages and Mortgage-Extra, which allows for topping-up payments.

National Irish Bank also produced a popular accelerating mortgage, in which the customer sets the extra percentage they wish to pay each year in order to make substantial interest savings.

In fact, virtually all the lenders will make some sort of special arrangement with a customer - whether it involves taking a payment break, borrowing against existing capital, or accelerating payments, but usually it takes time, considerable administration and extra cost.

"This product has taken over a year to develop," a spokesman for First National told Family Money, "with most of the time concentrated on getting the software programme right. Every branch manager can arrange this facility easily for customers at any stage of their loan and at no extra expense."

This product was introduced, says First National, because the rigidity of traditional mortgages no longer suited the changing work and lifestyles of an increasing number of borrowers. It is probably only a matter of time before other lenders follow suit.