When crime falls in London, who gets up to take the credit? Sir Ian Blair, Commissioner of the Metropolitan Police, of course. It is not that he made the relevant arrests himself, but his desk is where the buck stops. When a dangerous criminal is tracked down - congratulations to Sir Ian Blair. He didn't conduct the interviews or examine the forensic evidence - but his team did, and he takes the credit.
So when police officers shoot an innocent man several times in the head, Sir Ian needs to accept his share of the responsibility. When the failures that lead to this are systemic, all the more so. It's why you get the big bucks.
Trying to influence the verdict by evidence tampering when the Met itself is in the dock is, if anything, an even bigger offence. It the police will tamper with evidence when they are on trial, where is our assurance that they will not tamper with evidence when citizens are on trial.
Sir Ian is just dragging the story out. It is long past time for him to go.