Evil guardians Simon and Michelle McWilliam were starting eight-year jail terms today for a catalogue of cruelty to a four-year-old boy.

Judge Anthony Scott Gall told the couple: "You systematically set about abusing that little boy who wanted no more out of life than to be loved and to give love. You made a mockery of those wishes."

John Smith, described as a delightful, caring and polite little boy with a twinkling smile, died with 54 bruises, including three adult bite marks, on his body.

The boy, who teachers said had a bright future, was seen with injuries in four of the six months he spent with his prospective adoptive parents.

His penis was cut, his face burnt and his body bruised from head to toe, injuries seen by social workers who never once sought medical help for him.

John died on Christmas Eve 1999 from a severe brain haemorrhage, which experts testified was caused by blows normally seen on battered boxers.

Simon McWilliam, 41, and his 35-year-old wife Michelle, of Gardner Road, Fishersgate, Southwick, pleaded not guilty to cruelty and remained expressionless as they were each jailed for eight years at Lewes Crown Court.

The judge called them cruel and heartless and said it was hard to contemplate the fears their adoptive son felt in the months before his death.

The jury foreman wept openly after returning guilty verdicts on the couple.

What the jury had not been told was that the panel, which approved McWilliam as a suitable father, did not interview his previous two wives who said he was violent and should not be allowed near children.

An independent inquiry report has criticised social services for a string of failures and senior officials have suspended two social workers who were hoodwinked by the McWilliams into believing John was self harming.

Social services chief Allan Bowman has admitted serious errors and introduced changes recommended by the inquiry.

He has sent the report to the Victoria Climbie child abuse inquiry in London.

He said: "We let this boy down."

He promised the same mistakes would not be repeated.

The boy's natural parents, Marion and John Smith, are considering suing social services.

Mrs Smith said: "I'll never forgive them. John was a rainbow who touched everyone who knew him."

The boy's relatives wept with relief in the public gallery as the jury gave its decision after more than four hours.

Simon McWilliam's barrister, Jean McCreath, said John had been too much for the couple and "something quite dreadful" had gone wrong.

They had asked for help, maybe not in the right way, and Simon McWilliam had failed to see how out of control things had become.

There had been a suggestion that sentencing would be delayed for reports.

But McWilliam leant forward from the dock and tapped his solicitor's clerk on the head, whispering to him that he and Michelle wanted their punishments straight away to get the court process over and done with as quickly as possible.

Michelle's barrister, Stephen Kay, said his client was an outpatient at a psychiatric hospital and had been on anti-depressants throughout the trial.

He said the case had left its mark on her. "The external pressure and public interest have been very great."

The couple had been left "entirely in a corner and on their own".

Judge Scott Gall was less sympathetic, describing the case as harrowing.

"It is hard to imagine a worse case," he said.

He did not doubt the couple were genuine when they applied to adopt but the process had to be on their terms and the child had to fit their requirements.

He said: "Little John Antony was not to your liking.

"I regard you as equal partners in this dreadful case."

Susan Diplock, legal representative for John's natural parents, said they were "extremely grateful" to the jury, the judge, the Crown Prosecution Service and Sussex Police.

She said: "Their son was a quiet, happy, friendly boy. He was much loved by his parents, his grandfather and his foster parents, Ron and Pauline Mears, who described John as their 'little star'.

"Mr and Mrs Smith cannot begin to understand why their son was placed with people such as the McWilliams. Mr and Mrs Smith hope their son will now rest in peace."

City MPs today renewed calls for a change in the law.

The McWilliams were originally charged with murder but the case failed at the committal stage.

It was similar to a trial last year in which a Brighton couple were cleared of murdering three babies when the judge ruled it was not possible to decide which of the two carried out the killings.

It led to a campaign by The Argus for a change in the law. The then home secretary, Jack Straw, responded with a proposed new crime of killing by cruelty, carrying a 14-year maximum jail term.