I'm 16 and had sex with a 15 year old?

[Breezy]

So me and this girl got talking a few weeks back, she's just turned 15 at the start of December and I'm 16, but 17 in February. The other night we met up and one thing lead to another and we ended up having sex. She gave her consent and I asked her several times before intercourse if she was sure this was okay, which she said she was. Her ex-boyfriend persistently will not leave her alone, it's to my belief that he is on bail however I am not too sure. When she was 14 and he was 15 they had sex too, which is obviously underage. Her ex who will not leave her alone has been contacting me recently threatening to go to the police over having sex. I stupidly told him on the phone that we had done it, however he has no proof apart from a text saying 'she said she regrets Friday' which doesn't actually mention what she regrets or contains the word 'sex' or any other implication of it apart from the word 'it'. She didn't say she regret it by the way, that is just something I told him. I really don't know what to do about him either, I have asked him to delete my number and leave me alone however he persistently texts me telling me to block her on twitter and stuff, he's threatening me with violence, which in all fairness I have threatened him before, however for the past few nights I have not whereas he has told me people are on me. I really don't know what to do I live in England, as I said she gave her full consent. Me and the girl are still speaking and are fantastic terms with one another.