A sports therapist has been jailed for at least 31 years after being found guilty of murdering a retired lecturer by shooting him with a crossbow at his remote island home.

Terence Whall, 39, targeted Gerald Corrigan, 74, as his victim adjusted his satellite dish outside his house in Anglesey, north Wales, in what police described as a “medieval-style execution”.

Sentencing Whall to life, Mrs Justice Jefford said the murder was barbaric, adding: “You have deprived Mr Corrigan’s family of any explanation for what was a horrific death in which Mr Corrigan was completely blameless. For your own reasons you clearly had a plan to kill. Your arrogant belief that you could get away with murder was misplaced.”

The reason for the attack remains a mystery. The judge said it was “speculation” to say Whall was paid to carry out the murder or that it happened because Corrigan was about to report a fraud to police.

In emotional victim impact statements, Corrigan’s partner, Marie Bailey, and his children paid tribute to him and spoke of their continuing agony.

Bailey said: “Gerry was an exceptional man, a man of intellect, my best friend of nearly 30 years and he was also my carer. Gerry looked after me, always watching out for me. He had a big caring heart.”

She continued: “My Gerry was shot and murdered outside our home. The person crept up on him unaware and Gerry didn’t have a chance. His back was turned, he’s hard of hearing and he was not even given the opportunity to defend himself.

“My memories of that very beautiful home we shared is now tainted because of the way he was attacked so horrifically. I was asleep upstairs, unaware, I couldn’t defend him. I wasn’t there, I couldn’t help and I had always been there by his side, but not this time. That is something that is with me constantly.

“At the time of the attack I couldn’t understand why there was so much blood. Gerry hadn’t realised he’d been shot. I desperately tried to mop up Gerry’s blood with towels. I will never forget Gerry clutching at what I now realise was a terrible injury. The pain must have been excruciating.”

She has appealed to Whall to explain why he shot Corrigan. “I would like the person who murdered Gerry to think about what he has done, the consequences of his actions and the effects he has caused to me and to all of Gerry’s loved ones.”

The victim’s daughter, Fiona Corrigan, said: “I just had a nice average life before all of this. Now, I suffer from nightmares, paranoia and such severe panic attacks that I scream and have recently begun to self-harm by using anything sharp to scratch my arms and neck. The worst thing of all is knowing that I won’t ever get to hug my dad again.”

His son, Neale Corrigan, said: “It was so difficult to comprehend how this could happen to anyone, let alone my father, who was just a pensioner on a small, safe island in the middle of nowhere. How can someone choose to use such a barbaric weapon on an old man? Did they really want to cause him a slow, painful death?”

Mold crown court was told the bolt passed completely through Corrigan’s body, causing terrible internal damage, including bruising the heart. He was rushed to hospital but he died almost a month later of multiple organ failure.

A ballistics expert and pathologist concluded the shooter must have been only 10 metres away, probably in the field in front of the house and hiding behind a wall.

Whall, who also lived on Anglesey having moved there from east London, denied having anything to do with Corrigan’s death but police were able to establish that a Land Rover he used was near the victim’s home at the time of the shooting and on the previous night, when he scoped out the property.

Co-defendant Gavin Jones, 36, was convicted of conspiracy to pervert the course of justice and was jailed for five years. He was involved in setting fire to the Land Rover to try to cover up the crime.

Jones’s brother, Darren Jones, 41, was sentenced to two years and 10 months for arson, and Martin Roberts, 35, was jailed for two years and four months for arson. The two men admitted setting fire to the Land Rover, but the court heard that they believed it was part of an insurance scam.