A TEENAGER who killed a kind conservationist at Welwyn Garden City station to impress "happy slapping" peers has been given three years detention for manslaughter.

Callum Latham, 16, of Nettlecroft, killed Richard Topp, 36 - described as hard-working, caring and good - with a single blow to the jaw in front of his fiance Esther Williams.

The couple were returning to north London on January 8 after a visit to Mr Topp's brothers, Simon and Andrew, who live in Welwyn Garden City.

Latham struck him at the foot of the staircase to the platform, breaking his jaw in two places, St Albans Crown Court has heard.

Mr Topp fell against the bannister, suffering a fatal head injury. He was rushed to the QEII Hospital, but pronounced dead at about 2pm the next day.

Prosecutor Ann Evans said Latham had been crossing the passenger bridge with two friends when one showed him a film of a "happy slapping" attack on his mobile phone.

She said: "Callum Latham boasted he reckoned he could do that."

He aimed a pretend punch at Mr Topp as they descended the stairs together, danced round him on the platform and challenged him: "What's your problem?"

When his victim replied he hadn't got a problem, Latham launched the fatal blow before running off.

He was on his way to Hatfield Police Station to keep a bail appointment and was soon identified from CCTV footage.

Ms Evans said: "The effect on his Mr Topp's family has been catastrophic.

"Esther Williams witnessed this terrible event."

She said Mr Topp's fiance, a committed Christian, had since given Latham a Bible for his 16th birthday.

Latham has previous convictions for assault, shoplifting and burglary, and a violent record at his school, Richard Hale in Hertford, leading to expulsion.

Defending, Richard Wormald said his client, who has been on remand in Feltham Young Offenders' Institute, was filled with remorse and could not stop thinking about Mr Topp.

He has a good record at Feltham and wants to meet Ms Williams.

Judge Michael Baker, announcing a three-year detention order, half to be served in custody, said there would be an extended licence period of three years.

He said Latham's record suggested he posed a serious risk of more violent offending, although there were grounds for cautious optimism.

In a statement read out in court, Ms Williams said of Mr Topp: "I cannot imagine life without him.

"He was a terrific man. There is nothing he would not have done for anybody.

"We had an amazing time together."

Mr Topp's parents, who live in France, spoke amicably with Latham's mother and father after the hearing.

His father Mike Topp said: "I don't see the point of a huge sentence.

"It doesn't achieve anything with kids like that."

Mr Topp, who worked in a warehouse, was a keen conservation volunteer, and his ashes were spread on a meadow in Hampshire which he had helped restore.