Alhamwi, who settled in the UK with his family after being accepted for a PhD at the University of Manchester, co-founded the charity Rethink Rebuild Society, which supports Syrian refugees in the UK.

He was jailed in 2003 for political activism in Darayya, on the outskirts of Damascus. A secret military court sentenced him to four years imprisonment in Sednaya, the first seven months of which he spent in solitary confinement.

The underground cell let in no sunlight. Images from the prison this week showed bare, damp concreted spaces with a block to sleep on.

“It was very cold,” he recalled. “I touched the cell and found two blankets, so I put one under me and one over me. I never took off my jacket the entire seven months I spent there. The water was frozen. I would shower with frozen water.”

There was no connection with the outside world, he said. His father knew he had been imprisoned in Sednaya, but guards denied he was there. “When they took me out of the solitary cell, it was the happiest day of my life,” he recalled. Today, I still remember when they called my name — even though they took me to the public prison, not home.”

He was moved to the “ordinary cells”, which held groups of prisoners. He said: “We would be 24 grouped together, every cell accommodated roughly 20. At that time, in 2003 to 2005 when I was there, it was supposed to be the nicest time for Sednaya.

“In 2008, the prisoners tried to ask for some rights, and there was trouble in the prison. After that time they made it much tougher, and then after 2011, after the uprising it became a slaughterhouse.”

After the assassination of the former Lebanese prime minister Rafic Hariri in 2005, in which the Syrain government was implicated, the UN put pressure on Assad to co-operate. In a gesture of appeasement, he agreed to pardon 190 political prisoners. They included Alhamwi, who had served two and a half years of his sentence.

He was granted permission to leave Syria for a PhD in the UK. He said: “Then the situation in Syria worsened, and my father and brother were arrested. I knew I couldn’t go back.”

His father was arrested three times. “After the last time, we begged him to leave the country because the investigator told him, we will keep arresting you until you die.

“My brother was imprisoned with my father the second time. He also fled the country. But my father-in-law was taken from his home in July 2011 and he disappeared. For 13 years, we did not know anything about him, until July this year when the civil registry gave the certificate to my family-in-law, confirming he had died in 2014.”

• Inside Sednaya prison, Assad’s ‘human slaughterhouse’

Thousands of Syrians are still searching desperately for loved ones who disappeared under the Assad regime. “Their artificial hopes are starting to disappear now,” Alhamwi said. “They thought there were more floors, other spaces underground where their relatives might be. But they are discovering now that those floors don’t exist, and if their relatives are missing, then it is because they were killed.”

He urged the government not to rush decisions on Syrian refugees. “Many tens of thousands of refugees have no homes in Syria any more. My flat, my parents flat, all of it is destroyed completely. Half of my home town is destroyed.

“There must be careful thought about these issues, because it is not just about the political asylum. And on the political side, we don’t know yet about those who come to power — whether they will respect human rights. We hope they will be OK, but who knows if they will abide by what they are saying, or if they continue the abuses they did before?”

The road to healing for Syrians who experienced the regime’s brutality is long, he said, and added: “Sometimes you think you are OK, but then it comes back to you and you realise you are still not.

“I was watching the reports on the prison, and what they did to people in there, but I had to stop after a couple of minutes,” he said. “I don’t want to listen to these things anymore. I want us to look to the future.”