I think that 'it is never to late' is a ludicrous statement to make at this remove. Your man was convicted on the evidence available at the time which was obviously still potent years later on appeal - and so it remains.
You thinking something is ludicrous doesn't make it so.
My man is my Pete not JB.
Like all prisoners, no matter how henious the crimes they were convicted of, JB has access to the appeal courts who will grant leave to appeal if he's able to provide fresh evidence.
In this case the UK isn't known for its expertise in ballistics even today let alone 35 years ago unlike say the US where gun crime is routine.
I believe a reconstruction will make a compelling appeal point added to the two reasons for referral which relate to the blood and silencer evidence.
I would point out that there are many additional points other than a reconstruction.
The following gives some idea of a shooting reconstruction
https://youtu.be/R5JDhi4URds