This is because all 20,000 mortgage holders on its books at 31 December 2006 are in line for windfalls worth an estimated 15,000 each if, as seems likely, the building society is sold sometime next year. This should more than make up for the Nationwide's slightly higher rates. And once the windfall is in your pocket there is nothing to stop you switching to a cheaper mortgage lender.

Of course, there is always an outside chance of being overtaken by events if the sale happens before the end of the year. But late-comers have a better change of bagging a windfall if they sign up as borrowers rather than savers.

To open a qualifying share account today, you would need to deposit at least 20,000. And to get the windfall, your deposit must be at least two years old when the Nationwide calls a vote on shedding its building society status. So new savers have already missed the boat. Fingleton is coy about exactly when demutualisation will happen. He expects legislation paving the way for a fasttrack sale of the Nationwide to be on the statute books by the summer. Only then will the search for a buyer begin in earnest.

"If a buyer appears quickly, [the sale] could be done by the end of the year, " he says. "But we're not going to deal with anybody until the legislation is published and we've had a chance to see what's in it and to be sure that we can comply with it.

We prefer to deal with reality rather than speculation."

The building society is sensitive to criticism that, instead of dragging its heels until it got the legislation it wanted, it should have sold up years ago. By delaying until now, Fingleton says the windfalls will be much chunkier because of the spectacular profit growth posted over the last decade.

He takes pride in pointing out that the Nationwide's profits, which jumped 30% last year to 176m before tax, are bigger than those of Bank of Scotland (Ireland), Permanent TSB or the EBS, which will be the last remaining building society when the Nationwide is sold.

Now that the end is finally in sight, Fingleton is determined to keep up the momentum on a sale. "You can rest assured there'll be no delay on our part, " he says. "[The sale process] is well advanced from an administrative point of view."

But these reassurances are of little comfort to elderly people, who account for a big proportion of the Nationwide's 100,000 eligible savers.

They fear they may no longer be around when the windfalls are handed out. Even their children would lose out because, under building society law, membership dies with the member.

Putting a value on the Nationwide, and therefore on the size of the windfalls, is difficult because it is a building society like no other. "No other institution, except perhaps Anglo Irish Bank, would compare with the nature of our business, " Fingleton says.

"So it's very hard to put a value on it."

While home mortgages are the bread-and-butter business of other building societies, two-thirds of the Nationwide's 7.6bn loan book is made up of commercial loans. And the bias towards commercial increases year by year, with lending to property developers, typically for house-building, accounting for 70% of gross new lending of 3.6bn last year.

This is where the money is to be made, Fingleton says.

"By focusing on commercial lending, we're using our funds to best advantage because this type of lending is more profitable and has better margins, " he says.

To finance the booming loan book, the Nationwide relies heavily on retail deposits, which jumped by 1bn last year. The national network of 50 branches is the backbone of the deposit business, and Fingleton says these outlets could be a big attraction for potential buyers. "We're not big enough to compete in retail banking but we have the structure, the staff and the locations for anybody who does, " he says.

While home mortgages are increasingly a sideline business, it has generated a lot of bad press for the Nationwide in recent years, with angry borrowers using the annual general meeting as a public platform to air their grievances.

Fingleton given them short shrift. "They're the same handful of people recycled from year to year, " he says.

"You'll always have difficult customers, even if you charged them 0% interest.

But we seem to get more publicity for ours than anybody else."

With this year's AGM scheduled for 28 April, these dismissive words can only whet the dissidents' appetite for another showdown with Fingleton.