The beautiful bridge across the River Thames that so many of us love to hate is up for sale. With up to four million cars crossing every year, paying 5p each, could it be a snip of an investment at an asking price of £1.65m?

“Beat the Budget! Beat the Taxman! It’s a Win-Win Situation!”, runs the advert from agents HLL Humberts Leisure of Mayfair, London — and with good reason.

Revenue from the tolls is free of income tax, while the bridge itself, a Scheduled Ancient Monument, is free of inheritance tax, capital gains tax and stamp duty.

John Mitchell, of Humberts, said: “In the current economic climate, and particularly in the aftermath of the Budget with its new 50 per cent top rate of tax, we expect keen interest.”

This oddity of an investment opportunity came about thanks to what some might describe as an early example of a Private Finance Initiative (PFI) — similar, for instance, to the funding method of the M6 Toll Road, or the Severn Road Bridge.

In 1767 the fourth Earl of Abingdon privately funded a piece of public infrastructure in the shape of the Grade II listed bridge at Swinford, near Eynsham, which carries the B4044 (then the main road from Witney to Oxford) across the River Thames.

In return for his investment of about £5,000, an Act of Parliament, passed specifically for the purpose, stipulated that the bridge and its tolls would belong to the Earl and “to his heirs and assignees forever.”

The 1767 Swinford Toll Bridge Act was drawn up by Sir William Blackstone, then living in Wallingford, who made sure that abolishing the tolls would require another Act of Parliament.

As might be expected from one of England’s most famous jurists, whose textbooks are still studied by law students throughout the English speaking world, the wording was cunning; not least because of that “forever” word.

Most such early “pontage” arrangements contained a time limit designed to give the financier a fair return on his money and then disappear, leaving the bridge or road for free public use — but not this one.

Up for sale now are more than four acres of land, a pretty toll keeper’s cottage by the river, and the bridge itself.

At present, the bridge belongs to the two sons and daughter of Graham Smith, of Stratford-upon- Avon, who died in 2001.

Mr Smith bought it in 1985 for about £275,000, even though there was a strong campaign at the time to persauade Oxfordshire County Council to buy it and abolish the tolls, which cause commuter delays every day.

Michael Hawley, son-in-law of Graham Smith and spokesman for the owners, said: “We are selling it because there are now 13 family members — children, grandchildren, wives and husbands — all with different interests and aspirations and we want to present a united front.

“This is one of the final items in winding up the estate. And although the bridge is free of inheritance tax, we could still do with the money to pay the tax on other items.”

Now, once again, there is a clamour for the council to buy the bridge.

The Conservative county councillor for Eynsham, Charles Mathew, now up for re-election, said: “There is a widespread feeling the county council missed the opportunity to buy the bridge at the time of the toll increase in 1994, and also when it was sold in 1985.

“Now I think we should grab the opportunity with both hands — if the price is right.

“It’s got to happen. I want the tolls abolished to solve the major traffic problems they cause. I shall be pursuing the matter vigorously with the council.”

And Eynsham resident Jane Tomlinson, who runs the Scrap the Toll Campaign on her website, agreed. She said: “If the council cannot afford the price, it should buy it, keep the toll long enough to recover the purchase price — and then abolish it.”

Mr Hawley said: “Of course, the council is welcome to bid, along with any other potential buyers on the open market. And as owners they could decide not to take the toll if they wished.”

In 1994 the toll was increased with the permission of the Secretary of State for Transport for the first time in 225 years, from 2p to 5p for cars.

The tolls are regulated under the Transport Charges, etc (Miscellaneous Provisions) Act 1954, but to increase the toll, the bridge’s owner must apply to the Secretary of State for permission.

Mr Hawley said: “To increase the toll under an amended order to the Act, the owner must show that the money is necessary because of increased maintenance costs or inflation.

“I know how to do this now, the mechanics of how to do it, and I know that a new owner would be able to increase it, perhaps to 10p.”

Back in 1994, a public inquiry was held before the toll was raised to 5p. And when it was introduced it was decided that the extra 3p should go into a special fund to pay for a maintenance plan outlined by surveyors Gifford and Partners.

Mr Hawley said: “Since then we have paid out £750,000 in maintenance and our accountants at Grant Thornton have shown evidence of the 3p fund each year to the county council. Work on the bridge never stops, but we are exactly on target with our spending as envisaged back then by Gifford. There is still some money in the maintenance pot, and the purchaser will take that over.”

He will take over some other extraordinary rights, too. For instance, even now no one may build a bridge or operate a ferry within two miles of Swinford, in order to protect the tolls from competition.

Other odd clauses in the Act allow the bridge’s owner to seize the car of anyone crossing the bridge without paying, and a stipulation that should the bridge ever be closed, the owners must provide a ferry.

The name Swinford derives from Swine ford — a crossing for swine — in much the same way as Oxford was a crossing for oxen, and the bridge was originally built to replace a ferry. There is a story that King George III himself got wet when using the ferry — which gave added momentum to the bridge building. Whether true or not, Lord Abingdon made no money out of it. Indeed he ran into financial trouble by building a hostelry at one end of the bridge, which no one much patronised.

In 1955, the then Lord Abingdon succeeded in raising the toll to 5 old pence, pointing to the 1767 Act, which allowed him to charge a penny a wheel — and saying that the spare wheel also counted.

The late Mr Smith also owned the toll bridge at Whitney on Wye, Herefordshire, where the charge is now 50p. That bridge was sold in 2002 by Mr Smith’s estate.

The only other privately owned toll bridge in Oxfordshire is at Whitchurch where the toll is now 20p for cars, with local people, buying a computerised “Bridge Card,” paying a reduced rate of 13.5p. Now managers at the Whitchurch Bridge Company are applying to the Department of Transport to double it to 40p per crossing from July to fund major building works.

Marcus Mabberley, a spokesman for Oxfordshire County Council, said: “We have obviously been made aware that the bridge is for sale and will discuss the issue internally in due course.

“Public money must be spent appropriately and with due caution, and transport policies need to be framed as part of wider strategies that should be considered in the round, not just on the basis of one bridge in one part of Oxfordshire.”