A man who stabbed his partner and friend after finding them having sex on a canal boat made a desperate attempt to save their lives, a court heard today.

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William Cranston, 44, discovered Kay Morton and Paul Wilkins in the living room of his narrowboat after hearing noises as he slept.

The Sky TV satellite engineer "lashed out" at the couple, not realising he had a six-inch kitchen knife in his hand, a jury at Reading Crown Court was told.

Cranston said his 39-year-old partner and Mr Wilkins, 52, collapsed to the floor, and that he only realised he had stabbed them when he saw blood.

He called an ambulance and threw the knife from the boat into the canal at Stoke Hammond, Buckinghamshire, before attempting to revive the pair with chest compressions and mouth-to-mouth resuscitation.

Neither regained consciousness. Cranston admits manslaughter but denies murdering Ms Morton and Mr Wilkins.

He told police he and Ms Morton were still grieving the loss of their son when the incident happened in the early hours of September 7, 2008.

Cranston said: "We had been through a bad time this year after our son died."

Referring to seeing his partner and friend together, he added: "To see that is just a big shock."

The trio had been drinking with other friends at the nearby Three Locks pub the previous evening.

When they got back to the boat, they carried on drinking while Ms Morton made dinner.

Cranston went to bed between 10pm and 11pm, saying he felt "drunk and dizzy", because he had to wake up early the next day to get to work in Stevenage.

He said he did not know what time it was when he awoke and entered the living room to find Mr Wilkins, a friend of more than four years, with his trousers around his ankles.

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Speaking to officers hours after the stabbings, Cranston said: "I found Kay and Paul having sex on the sofa. Paul jumped up and came towards me.

"I can't remember picking up the knife. When he came towards me, I just sort of lashed out to find out what was going on.

"Literally five seconds later, they were both on the floor bleeding to death, and I've got a knife in my hand.

"I don't know what happened. It's just crazy."

Cranston said he was "shocked" by the scene, and that when Mr Wilkins came towards him with his hands in the air he "digged" at him while swearing at the pair.

When Ms Morton stood up, he pushed her, making her stagger backwards and fall under a table.

Cranston, who was wearing his work clothes as he stabbed the lovers, said neither of them spoke to him, but that he believed Mr Wilkins' raised hands gesture meant: "I'm sorry mate."

Describing the aftermath of the attack, he told police: "It's all just a blur. There was just so much blood."