A private detective accused of plotting to hold a stolen Leonardo da Vinci masterpiece to ransom was conned, a jury heard on Thursday.

Robert Graham, 57, was told a contract had been signed which protected him from prosecution if he helped return the art treasure to its rightful home in Drumlanrig Castle, Dumfreisshire.

But a Lancashire-based lawyer he turned to for help lied to him, claimed solicitor advocate John Keenan, defending in his closing speech at the High Court in Edinburgh.

Graham is one of five men accused of demanding a ransom of more than £4million for the safe return of the Madonna of the Yarnwinder painting.

The art treasure had been missing for more than four years after armed raiders stole it from the Duke of Buccleuch's stately home in August 2003.

Mr Keenan, asking jurors to acquit Graham, said he had been confronted by "a thorny problem" after a tip-off that he could get his hands on the painting.

He needed expert advice and went to solicitor Marshall Ronald to see how the picture could be returned to its rightful owner without breaking the law.

Ronald, 53, then began "a personal crusade" with series of exchanges with a man he knew as John Craig, believing him to be the Duke of Buccleuch's representative. Mr Craig was, in fact, an undercover detective.

The seven week trial has heard that Ronald also used the opportunity to negotiate a £2million cut for himself - without telling Graham or any of the other accused.

Mr Keenan said: "Unfortunately he (Graham) was let down and, worse than that, he was deceived.

"Much of what Marshall Ronald did was deceitful and dishonest but it has not been established that the things he was doing were done with the knowledge of Robert Graham.

"He was kept in the dark about the way things were progressing."

Ronald had also described Graham and co-accused John Doyle - partner in their private investigation firm - as "those two idiots".

Mr Keenan said the under-cover officer had also helped persuade Graham that a contract gave him immunity.

"Without the lies spun by Marshall Ronald and John Craig the return of the painting would not have happened.

"The only true conspiracy was between Marshall Ronald and John Craig to deceive Robert Graham."

Solicitor advocate Maurice Smyth, for Doyle, said the two private eyes were proud and delighted to be able to return the Leonardo da Vinci painting.

"Their intentions were pure. They were giving the painting back to the Duke," he told the jury.

Defence QC Jack Davidson, for Scottish lawyer Calum Jones, 45, described the story which had unwound during the trial as "a colourful tale" but insisted Jones had done no wrong.

Mr Davidson said it beggared belief that during the course of a meeting lasting little over and hour Jones had agreed to a career-wrecking conspiracy plan, putting his whole life in jeopardy, with people he had never met before.

On trial are solicitor Marshall Ronald, 53, of Highmeadow, Ravenscroft, Upholland, Skelmersdale; private investigator Robert Graham, 57, of Gawhill Lane, Aughton, Ormskirk, Lancashire, his private eye partner John Doyle, 61, of Summerwood Lane, Halsall, Ormskirk, Lancashire; solicitor Calum Jones, 45, of Kepstorn, Knockbuckle Road, Kilmacolm, Renfrewshire, and solicitor David Boyce, 63, of Clark Street, Airdrie, Lanarkshire.

They deny conspiring to extort pounds £4.25m between July and October 2007.  An alternative charge of attempting to extort the money has been dropped.