If your considering putting a complaint in, visit your local force website for details with regards to their complaints proceedure.
Make it known that you wish to make a complaint against a serving police officer.
Officer taking the complaint should be the senior officer on duty.
Complaint should be recorded, it may not be.
Go through the process of making the complaint, even if unsuccessful. Set proceedure to follow.
The force concerned has a legal obligation to consider the complaint made against an officer.
All else fails make a complaint through the IPCC. Thios will be passed back to the force in question to respond to, they have no choice but to respond to it.
Assuming that you have basic details of officer name(s) & number(s), for those involved. Failing that, times, dates & station to which you were taken to. From that can be worked out the officer(s) involved.
Also worth considering is a Freedom of Information request. I'd also ask for copies of the notebooks used by all the officers involved, to be included in that request. After all they've nothing to hide have they!
Process is slow, tedious & in my opinon designed to grind you down. Don't let it or them.
If you are using a solictor, get them to make the complaint & request on your behalf.
Best o' luck