A man who raped two young girls and sexually abused another two over a 20-year period was finally brought to justice by a letter written to an agony aunt.

The note, which was written by one of John Robertson's rape victims and not sent was found by a family member in 2002, who read the heartbreaking letter and police were called in.

The letter stated: "For the last seven years John Robertson has sexually abused me. I am ashamed and embarrassed. Can you help."

At the High Court in Glasgow, Judge Michael O'Grady QC told Robertson: "You have been found guilty of an appalling course of conduct."

Robertson, 45, will be sentenced later this month.