The starting point for sentencing for murder is 15 years. There are then guidelines (depending on severity) in respect to other starting points of 25 years, 30 years and "whole life" tariffs may be warranted.
On top of that mitigating and aggravating features will be taken into consideration when calculating a sentence - I think only a maximum of 5 years or 1/6 of the sentence can be knocked off for mitigating features. Aggravating features can include a sexual element or concealing the body afterwards, to name a couple off the top of my head.
Schedule 21 of the Criminal Justice Act 2003 - Determination of minimum term in relation to mandatory life sentence - is where it is all laid out. Typically a 'common or garden murderer' (if there is such a thing) gets a minimum sentence of between 10 and 20 years.
On of the biggest impacting differences between a sentence for manslaughter and murder is that someone released after serving a sentence for murder remains "on licence" for the rest of their lives after release ie they can be recalled to prison for the slightest of misdemeaners at any time until they die. They also must serve a minimum term before they can be even considered for parole, which they aren't guaranteed to get.
With a manslaughter conviction (unless given a life sentence) they will be released at the two-thirds or at worst three quarter point (I think thats still the case) of their sentence and then remain "on licence" only until the end of that sentence ie if they were given lets say 12 years - they may be released on licence after 8 years, then remain on licence until 12 years have passed and then thats it where being recalled to prison for any reason is concerned.
I believe that it is the case these days we have what is known as a 'standard determinate sentence' with a "compulsory half way release date" - so someone sentenced to 12 years would be 'let out' after serving 6. Although the rules are quite complicated and depend on this and that. There is a detailed explanation here - http://www.howardleague.org/francescrookblog/complex-rules-for-release-from-prison
Sentence wise for manslaughter a life sentence is discretionary (compared to it being mandatory for murder). There's no minimum - the judge decides, and sentencing is based on past precedent (ie previous cases) and formal sentencing guidelines.
Manslaughter is subject to a wide variety of sentencing - you can get life, or you can get probation - simply because there are different kinds of manslaughter in terms of both the law and the circumstances in which the homicide occured.