By Jeremy Armstrong It is 32 years since Sharon Henderson lost her little girl at the hands of a sadistic killer. Only now, a year after he was finally brought to justice, has she been able to cry properly for her precious daughter Nikki Allan. The campaigning mother turned detective to track down David Boyd. The jobless drifter was finally found guilty of Nikki’s murder on this day last year (May 12), 31 years after the defenceless six-year-old was stabbed 37 times. Boyd, who had a long history of sexual offences, also hit her about the head with a brick. A Mirror investigation revealed the t…