The battle has been conducted amid allegations of feuds and vendettas, and the threat of libel writs. As well as academics, the contestants include a housewife who says she passed up the chance to make a fortune, and a 79-year-old writer famous for her historical novels about Llywelyn the Great.
The disputed site, at the mountainside village of Aber near Conwy in north Wales, has lain for centuries under a handsome Elizabethan mansion with magnificent views across the Menai Straits to Anglesey. Wilfred Pickles, the broadcaster, stayed there in the Thirties. Sir Peter Scott, the naturalist, used it as a base for watching birds.
Its deeper history, linking it perhaps to the medieval prince who tried to drive the English from Wales, was unearthed by an English couple, Brian Gibson, an engineer, and his wife, Kathryn, an amateur historian, when they bought the house and 36 acres for pounds 225,000 five years ago.
The Gibsons said they had discovered evidence of medieval stonework under the mansion, known as Pen y Bryn, in a tower buttressed into its end wall and in a neighbouring 'gate-house' barn. 'I'm only an amateur historian, but you have eyes that see and the evidence we've accumulated is quite substantial,' Mrs Gibson said.
'Brian's an engineer, very down-to-earth and very good at seeing how structures are built. The tower pre-dates the house and has been called Llywelyn tower for generations.
'I think the present house is sitting on a medieval long house because the walls of a room we found under the floorboards are so thick and have little slit windows.
'There's also a little cottage next door, still known as the old chapel, which sits on much earlier foundations. A woman who lived there in the Fifties found some old papers in a cavity behind a fireplace and unfortunately burned them because they weren't in English or Welsh.
Then there are the windows in the gate house, which are early medieval and most likely came from the original long house.
'We did a lot of research at the Public Record Office in London and found a document dated 1303 recording repairs to the stone windows and iron bars in the long house. In Aberystwyth, Bangor, everywhere there were documents. Welsh history is more oral history than something you learn in school. People round here came to us with little books, little stories, bits and pieces, all of which led us to think this was the site of Llywelyn's court.'
But local experts, led by David Longley, director of the Gwynedd Archaeological Trust (Gat), refused to recognise this. In a report commissioned by Cadw, the Welsh equivalent of English Heritage, Gat insisted that the mansion was an 'interesting' property dating from the 16th century.
The report crushed the Gibsons' dream of having Pen y Bryn scheduled and preserved as a historic monument to Welsh nationhood. Having got into debt, they fell behind with the mortgage and now face eviction.
Enter David Austin, a medievalist and reader in archaeology at the University of Wales, Lampeter. Mr Austin has examined the site and has told Cadw that he believes that it is undoubtedly medieval and should be given immediate protection. Cadw, recently accused by the Commons select committee on Welsh affairs of neglecting Welsh monuments, has promised to reopen the case: 'If Mr Austin has new evidence we'd certainly review the question of scheduling the site.'
Mr Austin said: 'Just one look at the place told me the trust's report was way out of line. We may never be able to establish that it's the house of Llywelyn ap Gruffudd, but there's a strong possibility it was his court.'
One of Mrs Gibson's main supporters is Edith Pargeter, the novelist who, under the pen name Ellis Peters, created the 12th-century sleuth Brother Cadfael. She has also published a four-volume biography of Llywelyn, The Brothers of Gwynedd, based on detailed research in North Wales. Miss Pargeter, who has donated pounds 5,000 to a trust set up by the Gibsons to save Pen y Bryn, said: 'I'm very pleased and hope Cadw will take it over and look after it.'
Mr Longley, who became director of Gat a year ago after nine years with the organisation, has reacted angrily to Mr Austin's findings and wrote to him last week to tell him so. 'I've had a powerful, vitriolic letter from him,' Mr Austin said. 'In the last paragraph he accuses me of being unprofessional, basically, and if that were to go public I'd have to sue him. It's up to him to have that opinion of me if he wishes. I may have a similar opinion of him, given a certain report of his that I've read.'
Mrs Gibson said: 'We only came here because it seemed a perfect place to bring up our three daughters. Then the Welsh Office decided to put the A55 by our bottom fields and developers started offering huge sums of money to buy land for houses, caravan parks and a petrol station.
'We could have made the best part of pounds 2m but if we'd done that no one round here would have spoken to us again and we'd have been running away with the children trying to find the same sort of dream we'd moved here for. How would you explain that to children?
'David Longley's report was used against us when the building society took us to court for possession and we were pleading for time to prove Pen y Bryn was medieval and get it protected.'
Mr Longley last week defied Mr Austin to produce his evidence. 'Everyone accepts there was a medieval building somewhere at Aber but no one's been able to show that that complex was at Pen y Bryn,' he insisted.
'We're passionately interested in the search for royal court sites, including one at Aber. People are asking for the site to be saved; but questions do have to be asked.' Mr Austin, an English academic in a Welsh environment, believes there may be a hidden, partly nationalistic, motive involved. 'The Welsh historical establishment never have made out where Llywelyn's court is,' he says. 'Then some English amateurs come along and how do they react? I'll leave that question hanging pregnant in the air.'