Eryk Pelka haunted by the murder of his only son Daniel Pelka

At first glance it appears that Eryk Pelka has carved out a happy life for himself and his young family. A thin man in his early thirties, Eryk works long hours as a welder and locksmith. It's a job that brings in enough money to rent a small flat, where he lives with his wife and two daughters.

But when I met him at his flat in a block on the outskirts of the historic Polish city of Grudziadz it quickly became clear that Eryk was a man with a troubled past. He is haunted by a little boy he last saw in Coventry, England, in December 2008. This was his only son Daniel, who he remembers as a cheerful, chubby little boy with blonde hair, who he left in the care of his partner, Magdelena, when their relationship fell apart.

Read: Daniel Pelka's mother and boyfriend guilty of murder

Daniel was killed by his mother and her lover, Mariusz Krezolek, after months of mental and physical abuse that left his emaciated body covered in injuries. He was taken to hospital in 2012 with fatal head injuries at the age of four but weighing just one and a half stone.

Eryk only discovered his son was dead when a relative heard about it and came to tell him. He says that he tried to keep in touch with his child after he left England but Magdelena, or Magda as he still calls her, would not allow him to speak to Daniel or send him any photographs.

He has just one picture of a three-year-old Daniel, smartly dressed in a little jacket, which he found on the internet after the murder trial began. This has been printed and placed in a photo frame, sandwiched between the two step sisters he never knew; five-year-old Ola, who Eryk adopted when he married her mother Karolina on his return to Poland, and their child, Klaudia who is now two. Both children are obviously adored.

After discovering what she and Mariusz did to Daniel he says, "I feel very bad, I want to kill Magda, I want to kill Mariusz. It's not human that someone would kill a little boy - why?" and he trails off shaking his head.

Eryk blames himself for not doing more to ensure Daniel was being properly cared for by his ex-partner and her new boyfriend. He says that when he and Magdelena separated she appeared to be a good mother. He tells me that she always provided enough food for Daniel and her older child and they were always clean and well dressed. She might occasionally scold the little boy but only if he had misbehaved, and she never hurt him.

Eryk moved to England in 2004 in search of work and money, content to do anything that would earn him an income. His jobs included working in a warehouse, at a chicken factory, as a lorry driver and a crane operator. He met Magda in Poland and took her back to Coventry with him in 2005.

But after three years they were continaully arguing and she started a relationship with another man. Finding it difficult to get work, Eryk moved back to Poland and started a new business with a friend.

Hearing about the abuse Daniel suffered, being punished by being force fed salt when he tried to steal food from bins at school, Eryk looks more miserable than angry.

He can't bear to read about the details but his wife Karolina follows the murder trial closely on the internet.

Together they have tried to come up with an explanation as to why Daniel was singled out for such cruelty but his siblings remained unharmed.

"I don't think they wanted him", Eryk tells me, "but why Magdelena don't tell me? I would take him to be with my own family. I have two little girls, I would have found a way to bring him, he was my little boy."

I want to know if he thinks that Magdelena is guilty, can he really believe the woman he once loved was capable of doing that to a small child? He is certain that she must share equal blame with her boyfriend Mariusz and Eryk says that he wants the pair to feel the same pain as they inflicted on Daniel.

It is too late to save Daniel but Eryk and Karolina want to bring his body back to Poland. It is very expensive and they don't have much money but they are vowing to raise enough to bring him home. They want Daniel to rest close to them, to be in a grave visited by those who loved him rather than remaining in a country where he lived such a short, sad life.