The parents of an outstanding student whose killer could be released from prison after only two-and-a-half years have called on the Prime Minister to have sentencing guidelines explained to them.

In a hard-hitting letter, David and Margaret Brown, call on Tony Blair to "get tough on injustice and tough on the causes of injustice".

They feel the justice system failed their 21-year-old son Patrick, who died a year ago. Patrick died after labourer Christopher Woolley, 24, rained down punches on him, sending him careering down an embankment to land head first on concrete below.

Woolley, 24, of Bek Road, Newton Hall, Durham City, was given a life sentence for manslaughter at Sheffield Crown Court this week.

But Judge Bennett ruled he would serve a minimum of two-and-a-half years before being considered for parole.

Mr Brown points out to Mr Blair that Woolley had previously assaulted students from Durham University, in almost identical circumstances.

And he served only 18 months of a two-year sentence after nearly causing a student's death.

Mr Brown writes: "An inadequate sentence for his earlier occasion of violence directly allowed an unreformed, violent thug to kill our son.

"Imagine, therefore, in those circumstances our horror to learn that at Sheffield Crown Court Mr Justice Bennett passing a sentence of life, but with a tariff of only seven years.

"He further allowed a possibility of parole after three-and-a-half years ie two-and-a- half years from now."

Mr Brown of Darras Hall, Ponteland, Northumberland, is imploring the Mr Blair to have a legal officer explain to them guidelines issued to judges when deciding sentences of manslaughter.

He asks: "Just how bad must a case before an absolute minimum of five years is served behind bars for justice (and an opportunity for reform) to be achieved?

"The police say there are such guidelines, but they have never seen them and cannot help us."

Pledging to campaign relentlessly for redress, Mr Brown writes to Mr Blair: "Because of our your own championing for the pre-eminent role of family life in building a good society, we hope you will help us rebuild our family life by finding justice and peace of mind."