Bentley was also already in the custody of the police when the shot was fired.
Ralph McTell wrote a song about it; June Tabor covered it:
In 1998, according to this, Bentley was pardoned (46 years after his death)
In relation to "joint purpose", I saw a CCTV clip on TV the other day, in relation to the US. Two guys (at least one of them armed), raided a shop. The shopkeeper had his own gun and shot at them, killing one of the thieves, and the other thief - who may even have been unarmed - was done for murdering his friend.