Brefugee
If there aren't enough grounds to arrest - then in order to get their account, they can be offered an opportunity to be vol interviewed. This is to get their side of the story. Otherwise, we only have the complainants side of the story and nothing against it. Therefore the person would be charged or summonsed to court.
if they are interviewed - they may give an account which means there is no need for a court case. Their account maybe corroborated for example.
They can go 'no reply' - but if they are asked similar questions at court as to what they asked in interview, the court then can draw an inference. Basically why have you give a response now but not when you were interviewed.
There must be an offence there in the first place though - whether criminal (theft, burglary) or another offence that's not a crime (drink drive etc)
To arrest someone, you must have a necessity to do that (ongoing violent incident, fear for a persons safety etc). If they refuse a vol interview - you're still going to need a necessity to effect an arrest.
At the end of the day, you tell them at the start of a vol interview that you can leave at any point - you can't then arrest them. It won't be a vol interview then would it.
If you simply don't turn up for a pre-arranged vol interview, you can go down the lines of saying that a person is evading the judicial process and is a 'flight risk'.
So if you have a DV assault where A assaults B. You ask A to attend for a vol interview but they don't turn up. We can't just leave it at that. We have to make some attempts to get their account - otherwise do we just tell B that we tried to interview them, but as they didn't turn up there is nothing we can do.
If A does turn up but refuses their opportunity to be interviewed or leaves at the start of the interview - then we can't use the 'evading the judicial process' line to arrest, because they've fulfilled the request by attending for the interview. They have exercised their right to leave. So the case file will be processed to court with only B's account and nothing to counter it.
Hope this helps