Social workers were investigated by police after it emerged that key files relating to an alleged paedophile ring were doctored, a court has heard.

Ten 'seemingly respectable' people - including six women - are standing trial at Norwich Crown Court accused of subjecting two boys and three girls in Norwich and London to sexual and physical abuse over more than a decade.

Much of the abuse is said to have centred around Marie Black, 33, from Norwich, who denies 26 offences, including four counts of rape and two of conspiracy to rape, at Norwich Crown Court.

All of the defendants, aged between 31 and 85, deny abusing the children, saying the allegations were concocted by Norfolk County Council's children's services department.

Yesterday Gail Barnard, a senior council social worker, said the children had described being abused at sex parties and rewarded with certificates carrying slogans like 'secrets are good' and 'do not tell anyone'.

However, the court heard that the trial had originally been due to start last year only to be delayed when prosecutors raised concerns over changes made by social workers to statements taken from the children. This resulted in Norfolk Police launching an investigation into alleged misconduct.

Sarah Elliott QC, representing Black, told the court that at the time the county's children's services department had recently failed an Ofsted inspection, being ranked 'inadequate' in all areas.

Ms Barnard was asked if she had been questioned over claims she told a foster agency supervisor, Malcolm Blissett, to 'tidy up' reports in the case.

It was alleged she told him to remove leading questions and put some of the apparent claims by children into direct speech in order to comply with professional guidelines.

Ms Barnard replied: 'I knew there were complaints made against me.'

In 2010 police dropped their original investigation into the abuse saying the sheer number of allegations made the claims 'implausible', the court heard.

Ms Barnard is then said to have carried out her own investigation, including discussing the case with a chiropractor who had contact with one of the children, contrary to the guidelines.

Ms Elliott said: 'You were aware there was no ongoing police investigation. Did you decide to carry out your own investigation?'

Ms Barnard said: 'I wouldn't call it investigating but I did make inquiries.'

She also failed to tell police she was a patient of the chiropractor - despite allegations that the chiropractor was involved in abuse.

'I was embarrassed that I had had conversations which were too close for comfort,' she added.

Ms Elliott asked Ms Barnard if she had been aware of the Orkney, Cleveland and Rochdale satanic abuse scandals in the 1990s which saw children wrongly removed from their families. Ms Barnard replied: 'Yes.'

These cases brought about major changes in social work practices, including the way in which children should be questioned about abuse and how data is recorded and Ms Barnard confirmed she was aware of these guidelines.

Opening the trial, prosecutor Angela Rafferty QC accepted mistakes were made by social services, including Ms Barnard unwisely 'getting too close to the case' and 'interfering with documents'.

But she said any suggestion the allegations were made up was 'nonsense'.

'The foster carers and social workers in this case are going to be heavily criticised by those who defend,' she added.

'It is likely to be said that they are the ones who are responsible for making up allegations against the defendants.

'However, the idea that it is somehow them who planted these memories or got the children to make up these stories is an attempt to deflect you from the dreadful truth.'

Black also denies charges including neglect and ill-treatment, sexually assaulting children under 13, conspiracy to cause children to watch sexual acts and causing child pornography.

Nine others are accused of offences including rape, child cruelty, causing children under 13 to engage in sexual activity and sexual assault.

They are Michael Rogers, 53, from Romford; and Jason Adams, 43, Carol Stadler, 59, Anthony Stadler, 63, Nicola Collins, 36, Andrew Collins, 52, Judith Fuller, 31, Denise Barnes, 43, and Kathleen Adams, 85, all from Norwich.