I don't know the ins and outs of the law, but I wholly agree with pp about getting a solicitor to accompany you, regardless of your status as suspect or witness.
I was invited to the police station 6 years ago to attend a voluntary interview as I'd been present at a gathering where someone had accused another of a criminal offence.
I was so incredibly naive. I thought a "voluntary interview" would be some sort of informal affair where the police would take a statement and ask some questions about my version of events.
I couldn't have been more wrong. It was utterly horrific. My version of events wasn't matching up to what the police had clearly already decided happened that night - and I was subject to what can only be described as an interrogation. One of them was being the 'bad cop' - thumping the desk, raising his voice, and trying to trip me up with an extremely hostile line of questioning.
The other was trying to act 'good cop', had a gentler tone, but saying things like "Right, come on Jock, just tell us the truth and you won't be in any trouble". I was telling the truth, but neither believed me.
I was in there 2½ hours, and it was the most awful experience.
The case got referred to the CPS, and heard in the magistrates court nearly two years later. I gave evidence that day, suspect found not guilty.
I will never trust the police again. I would never call on them for help, and I hope that I'm never in a position where I need to be in contact with them ever again in my life.