Well, it seems that you know better than those who know. There is a document/letter here -I can only think it relates to COLP- which says it understands she was.
You mean the document that said they decided not to prosecute? The decision was made late 1985 or January 1986 and far from saying she was granted immunity if she agreed to testify it declared she would not be prosecuted because they had no basis to do so. How does notifying her they had no basis to charge her translate into testify or we will charge you?
From such document (worst typing errors fixed):
"Julie Mugford herself is 21 years of age and is described as- as intelligent young lady, presently studying for an honours degree In education at Goldsmith's College, London. To her detriment however, Julie Mugford has now confessed to the police in a third statement dated 10th September, 1985, her Involvement in drug abuse, selling about £100 worth of cum able, her involvement din a burglary and cheque frauds. All these offences appear to have been committed since her involvement with Jeremy Bamber and appear to have been at his instigation.
The Director was obliged to consider the question of prosecuting Miss Mugford for her involvement in the above mentioned matters , but decided against this course, bearing in mind, in particular, the following
a. to a greater or lesser degree, all the offences committed, by Miss Mugford were as a result of her being, corrupted "by Jeremy Bamber,
b. save for her admission, there is no direct evidence against Julie Mugford to justify her prosecution for the drugs offence . As a first offender following the policy adopted by the Essex Police in drug matters, she would have received a caution.
c. as to the burglary, Julie Mugford's involvement, save for an unsuccessful attempt to get the key hanging behind the door,, appears to have been that of a bystander.
d. so far as the cheque offences are concerned, both Mugford and her accomplisee Suzann Battersby have repaid the sum lost by the bank, who were the losers, have stated that they would not support a criminal proceeding."
It was a decision not to prosecute not a grant of immunity.