Technicalities that have allowed people to get away with murder or manslaughter? UK?

I think some trials have collaped when people have skipped the county rather than give evidence, I believe the famous London 'Twins', did get away with murder on several occasions when they played off the police and others to either take the rap themselves or to lose vital evidence, that was back in the 60/70's though they got too cocky and ended up paying for it.