When a person needs to be interviewed by police in relation to a criminal allegation, they can either be arrested and taken to a custody centre, or the police can invite them into a police station for a voluntary interview.
When a person attends an interview at a police station as a voluntary attender, they are there voluntarily and are free to leave at any time.
Anyone interviewed by police either under arrest or as a voluntary attender, no matter how serious or trivial the allegation, can have and use the services of the duty solicitor. That service is completely free. You can request the duty solicitor, and should inform the police officer who will carry our the interview of your request before you attend, so as to avoid any undue delays at the station.
When you and the solicitor arrive at the police station for the interview, the solicitor will speak with the officer carrying out the investigation, and obtain what is called advance disclosure. That means they will ask the police for details of the allegation and what evidence they have.
Next, the solicitor will speak privately with you and explain what they have been told and then obtain your instructions. The solicitor will then advice you on whether to answer questions in interview, or to submit a pre written statement or whether to go no comment.
Once the interview is over, the officer would normally say that the matter would still need to be investigated further and then a reviewing officer would check through everything. A file could then be sent to the Crown Prosecution Service if the police think there is evidence for a case. The CPS would then make a decision on whether to prosecute or not.
I would recommend asking for the duty solicitor as stated above.
From what you have already said, it really is pointless going no comment. They would already have his statement and the evidence ie massage you sent. No comment could well lead you to being charge to court for the offence.
What happened to you by him although it took place between the same two people, it is separate offences.
Certainly explain what you did and that you were drunk and that you don't recall doing it apart form seeing the message. Explain what happened to you as well.
That would be my advice.
However, ask for the duty solicitor, they will obtain advance disclosure from the interviewing officer and will advice you face to face what to do once they have received details from the officer.