“He told her not to and also told her that no one would believe her.”
Morrell was also ordered to sign the sex offenders’ register, where his name will now remain for the next 10 years.
Five other counts, four of indecent assault and one of rape, relating to when the victim was aged between 10 and 12 years old, were ordered to lie on file.
Mark Harries, in mitigation, said the families of all those involved in the case had been “shattered” by the “revisiting” of the events decades later.
He said: “Not withstanding the seriousness off these offences, they took place an extremely long time ago and all of the parties involved in this case are different people now from the people they were then.
“The victim was then a child but the defendant too was only a matter of a few years older.
“He too can be regarded as a child.”
On sentencing Judge John Wait said he had found the case “extremely difficult”.
He said: “These are very grave offences which took place over a substantial period, many months if not more, and at a time when your victim was aged somewhere between seven and nine.
“Had you been an adult committing these offences against a child the appropriate sentence would have been very long.
“It is a principle of sentencing that offences do not become less serious simply by the passage of time but while more than 35 years have passed since these offenses there are a number of factors that have to be borne in mind.
“Firstly the person who committed these offences was not Neil Morrell the grandfather but Neil Morrell the 15- or 16-year-old boy.
“And I have to bear in mind that you would have been dealt with at the time within the youth offending sentence regime.”
Judge Wait said he also took into consideration the fact that Morrell himself had been a “victim of abuse”.