Firstly, although gormless has not suggested otherwise, to avoid any confusion Derek Bentley was guilty of murder. Even his family accept that he was properly convicted. Their argument was over the death sentence imposed upon him (but not his under-age accomplice Christopher Craig) and they based this argument on his allegedly low mental age.
As has been said, if �Life� sentences were just that � imprisonment for the rest of your life � then this argument may not manifest itself quite so frequently.
When the abolition of Capital Punishment was debated in the late �50s early �60s, the alternative put to the public clearly was that full life sentences would be available to the courts. The courts, in their turn, would be expected to use them routinely in the same way as the death sentence was routinely imposed. This was a clear expectation upon which MPs voted. (You don�t have to believe me � just look up the relevant Hansard publications).
Nobody needs me to tell them that this expectation was very quickly managed out of the judicial system and we have today situations where murderers are serving as little as five years of their �Life� sentence. The average served (not �recommended� by the trial judge) is, I believe, a little under nine years. Yes, they are all liable to be recalled at any time to resume their sentence should they re-offend, but very few of them are.
Penalties imposed for murder in the UK are, by and large, unduly lenient. Capital punishment cannot be re-introduced in this country even if all the electorate, MPs, Ministers and members of the House of Lords wanted it. Not unless we repeal the 1998 Human Rights Acts and withdraw from the European Convention on Human Rights. This is not likely to happen so the issue of the length of �Life� sentences needs to be addressed.