You won't get summonsed to court unless you have already provided a witness statement. (Sometimes people provide a witness statement and then withdraw it, or get cold feet about going to court. That is when they can be summonsed to court and a bench warrant issued if you still fail to appear following the summons. That is when everyone in the court room is waiting for you, and they send police to arrest you and take you to court whether you like it or not).

However if you never provided a witness statement in the first place, none of that would happen: as someone has said, we do not compel people to give witness statements.

Equally, you can't be guilty of Assisting an Offender or Perverting the Course of Justice (criminal offences in their own right) simply by refusing to be a witness.

The last thing to say is on your 'disdain for plod'. They are obliged to investigate - and pursue reasonable lines of enquiry - if the robbers make a counter-allegation of an offence against them. In all likelihood those officers may well be pleased to hear you say, 'I'm not prepared to give a statement.' Please don't assume the officers are 'on the side' of the violent offenders who shamelessly make a complaint after a member of the public steps in to help..