Two private investigators from Lancashire were cleared of a conspiracy to extort £4.25m from the estate of the Duke of Buccleuch for the return of the Leonardo da Vinci painting The Madonna of the Yarnwinder.

The verdicts of ‘not proven’ against Robert Graham and John Doyle followed a trial in Scotland which “should never have gone to court”, according to Robert Graham.

John Doyle went further: “I do believe we are entitled to a reward,” he said. “What we did was to bring back a culturally-significant masterpiece, which is something neither the police nor the insurers could do.

“We brought it back and we have been through two-and-a-half years of hell since.”

The case against Lancashire solicitor Marshall Ronald was also found ‘not proven’, while two Glasgow solicitors were found not guilty.

The case was brought following a police ‘sting’ operation when the two investigators made inquiries via Marshall Ronald as to the payment of a reward for finding and returning the painting, which was stolen in 2003 from Drumlanrig Castle. All the men claimed all along to have been acting in good faith.