Six or seven years?

I do not understand the length of sentences handed down to Mark Masters and Sean Thompson who were found guilty of beating to death Gavin Waterhouse. The judge said the three teenagers who beat Mr Waterhouse to death were typical of the "young, drunken, violent yobs who hunt in packs, and of whom the public are sick and tired.". Judge James Stewart described the attack as "revolting, gratuitous...degrading."

This seems like murder to me, but the two youths were found guilty of manslaughter - one receiving a sentence of seven years in a young offenders institution, Thompson six years. Could someone explain why they were found guilty of manslaughter not murder and why these sentences are thought adequate.