Prosecutors say Stephen Seddon drove them into a canal to collect £231,000 inheritance, then shot them when they survived

A man staged a car accident in order to try to kill his parents – and shot them dead when they survived the crash against the odds, a court has heard.

Stephen Seddon, 46, known as Nic, plotted to murder his mother and father last year to cash in his £231,000 inheritance, a jury at Manchester crown court was told on Wednesday.

The married father-of-three was portrayed as a hero in the media when he claimed to have helped rescue his parents after driving them into a canal in Manchester in March 2012.

"He was happy to perpetuate this myth," said Peter Wright, QC, prosecuting. "He enjoyed his moment in the sun as the press portrayed him as the hero of the moment."

The praise was "music to his ears", said Wright, "but did little to alleviate his financial problems".

It was then, said Wright, that Seddon "needed to resort to a more certain method by which he could bring about their deaths – but one by which he could maintain that the deaths were a result of a terrible tragedy rather than a double murder by an ungrateful son".

The jury heard Seddon was in such dire financial straits prior to the murders that his parents – Robert Seddon, 68, and Patricia Seddon, 65 – had taken out a mortgage on the home they owned outright in order to buy their son and his family a house in Seaham in County Durham. They also gave him £38,500. But, the prosecution alleged, it was not enough for Seddon. "He had other plans," said Wright.

According to the prosecution, it was no coincidence that on the day Seddon drove a rented BMW into the Bridgewater canal he was carrying a knife and a steering wheel "autolock". These possessions allowed him to cut his seatbelt and smash his way out of the driver's seat as the car started to sink with his parents in the back and his disabled 17-year-old nephew in the front.

Seddon later claimed he always carried the autolock for "sentimental" reasons, said Wright, "because it had been given to him by a family member who had died".

But according to the prosecution, it was one of many clues that he planned to murder his parents by drowning them in the canal.

They only survived when members of the emergency services reached the scene and managed to pull them out of the car. Seddon's mother was unconscious and paramedics had to use a defibrillator to revive her.

Witnesses reported seeing Seddon standing on the roof of the car, apparently kicking in the windows as it sank.

Seddon's nephew, Daniel, who has severe learning difficulties, had managed to escape. Daniel had been cared for by Robert and Patricia Seddon, his grandparents, since his mother – Seddon's sister – died in 2008.

The jury heard that following the "accident" Seddon gave "many and varied" accounts of how it had come about. To some people he claimed he had hit a brick in the road – a scenario an accident investigator later ruled to be "highly improbable". To others he claimed he recalled having chest pains en route to what he said was a "belated Mother's Day meal". He said he must have blacked out, losing control of the car.

It was only when Robert and Patricia Seddon were discovered shot dead in their home in Sale, south Manchester, on 6 July last year, that detectives started to see the alleged car crash in a more sinister light, the court heard.

Their bodies were found two days after their murder, forensic experts believe. They had been shot at close range with a sawn-off shotgun. No possessions had been stolen from the house. The gun had been placed in the lap of Robert Seddon by the murderer, said Wright, "taking his right hand and placing it on the weapon in order to give the impression that he shot his wife and proceeded to take his own life".

The murderer was their only son, said Wright. "He wanted to lay a false trail. A trail, we say, he hoped would lead away rather than towards a man with a considerable motive to kill these two people."

The prosecution's case is that Seddon knew his parents made a will in October 2009, naming each other as beneficiary if one of them died, with their assets valued at £356,000 and after liabilities, an estate worth almost £231,000. But if they both died, said Wright, Stephen Seddon "got the lot". And that, said the barrister, is "why they both had to die."

Stephen Seddon denies murdering his parents. The case continues.

• This article was amended on 22 February 2013. The original referred to Stephen Seddon's parents as "elderly" in contravention of the Style guide.