Hi pixiefrog. You have motivated me to register after 6 months of lurking. May change my name at some point though. Random choice due to alt-J on the radio.
I'd back up what Purple2012 says. The police were very good with me and when I didn't want to prosecute left it on file saying to contact them if I changed my mind.
In my case my abuser was my father who I hadn't seen in 30 years so my motivation was very different from yours. I wanted to know he didn't have access to children and that if anyone else had reported him over the years that I could help their case.
The initial phone call was very difficult. I was having counselling but it didn't occur to me that anyone else could report for me. I asked for a specialist in sexual abuse in a strangled voice and was transferred straight away. I then spoke to a very matter of fact female detective and she was patient but unemotional which worked well for me.
This was followed up by a video interview. Again very difficult and emotional but kind of an exorcism for me. They are trying to get the maximum amount of detail from you and check consistency so the questioning was very repetitive. I think maybe they switch to this as a relief from the gory details. So they wanted exact position of body parts which even now makes me cry to think of. But they would then switch to asking about position of furniture in the room or who else was in the house and these more mundane details were a relief to talk about after the other.
The evidence was then passed to the police in the area the abuse took place who tracked my father down. I had phone contact with the officer in this area and also email which I found easier since it didn't matter if I cried.
The police in the area where my father now lives then interviewed him. I spoke to an officer on the phone about this after. Think it was the officer from the area where the abuse took place. She asked what I wanted to know and I said just that there had been no other complaints and there hadn't and that he didn't have access to children. She let slip that he was an alcoholic and implied he was so far gone no one would let kids anywhere near him. Result.
I had told them that he had abused my sister over a much longer period and they did ask if I could get her to speak to them. She said no but was very happy to hear my father was in a bad way. Since I didn't want to press charges and I'm guessing it would have been hard to get a conviction on the evidence they asked if I was happy to leave my allegations on file and for them to take no further action at the time.
For me at that point I had achieved what I wanted and I then wanted to focus on working with my counsellor on sorting out my complicated feelings towards other family members.
So for me it worked well. I got closure. I didn't have to go to court. I had some control and my abuser lost some. He knows that he hasn't completely got away with it. He probably wonders if my sister will report him and he would get charged. I like to think that he worries about that happening but he probably just has another drink.
On a note of caution - I had been slowly falling apart for a long time before I went for counselling (best money I spent in my life ever) so the process of reporting didn't make much difference. I was already off work and crying most days. I don't think I could have gone into work the day after the police interview and each contact with police was hard.
That was about 3 years ago and I have changed job and put myself back together. I'm not quite who I want to be yet but I feel like I'm getting there.
Good luck with your journey pixiefrog.