Mother reported me for harassment because I emailed her and I've now got a warning?

I haven't been in your situation but I can offer sympathy and a welcome to the site.

I figure the police were obligated to come around based upon your mother's complaint but I'm assuming you were not arrested. As for the other end of things, moving from warning to action can be an agonizingly slow process. But an investigation (e. g. like the visit you got) is necessary in order to determine whether there's a real complaint. At least in the United States, prosecutors generally aren't going to go after matters that they don't think they can win (they'll offer a plea bargain when they think a case is a loser or would be expensive to investigate and try).

You might want to speak with a lawyer (a solicitor in your case). Explain what's going on and ask about what appears to be a retaliatory action by your mother. I don't think you should get into a he said, she said type of an argument and escalating war, but if you are concerned that your mother will further try to ruin your reputation, then you should take some steps to protect yourself.

You do need help and support, and there's only so much you can get on this site. I echo what Jespah has posted. Google "Survivors of childhood sexual abuse" to find a support network near you. There are plenty out there, and if you talk to someone with similar experiences to your own, you won't feel so alone.

Good luck.

You said: "Any reassurances, help and support is welcomed and appreciated "

Well, it certainly is NOT going to come from your abuser, so stop trying to get validation from her.

Join a group where you can freely talk about these issues and learn to move on.