The father of a North Somerset mine expert who was captured, executed and then crudely cremated by Cambodian rebels believes the four men jailed for his son's murder "should never come out".

Christopher Howes, 37, from Backwell, and his interpreter, Houn Hourth, were abducted by Khmer Rouge guerrillas and then killed while working in north-west Cambodia in 1996.

Mr Howes was advising the Mines Advisory Group, a British-based humanitarian mine-clearing organisation, when he was seized.

Three of the captors were jailed for 20 years on Tuesday by a court in Phnom Penh for their part in the murders. A fourth is starting a 10-year sentence, while a fifth man was acquitted.

The court heard that Mr Howes was offered a last meal of forest fruit before he was murdered and then burned on a diesel-soaked woodpile.

His father, Roy Howes, 85, who struggles to speak due to illness, said: "I'm delighted. They are wicked men – the very worst. I hope they never come out, I hope we will never see them again."

Christopher's sister, Patricia Philips, said: "My father and I welcome the verdict of the court and feel that at last justice has been done.

"Although we have never sought revenge, we are pleased that the murderers have been brought to account. I am just sorry that mother, who sadly died in 2007, has not lived to see that justice has finally been done.

One of the defendants, Khem Ngun, admitted that he was responsible for transporting Mr Howes to the execution site, but denied he had prior knowledge of a plan to kill him. Khem Ngun, Loch Mao and Put Lim were present when Mr Howes was executed in the middle of the night in Anlong Veng, the court was told.

They were each sentenced to 20 years in prison. Sin Dorn was convicted of being involved in the early stages of the abductions and was sentenced to 10 years.