I thought I'd come back and update those of you who were kind enough to offer advice.
He has had access to his daughter stopped by social services. The police have spoken to him and warned him off. All of that and he still refuses to stop seeing dd.
I have given her old phone - which she stopped using around March as the screen was broken - to the police. They are trying to determine if anything at all happened between them before dd turned 16.
A detective came round last night to speak with her. She allowed him to go through her phone - photos and messages between him and her - and there wasn't really anything of concern on it. Although there were some messages where he would say "let's talk on the phone" which the detective found odd and feels that he was trying to stop leaving a "trail" as he put it.
She denies they have had a sexual relationship, although admitted that they have "kissed passionately" once, which was last weekend.
It did come out though that dd had slept with her previous boyfriend. As she was under 16 at the time, she was not able to consent so he wants to know if I want the previous boyfriend charged with "rape". I have spoken with dd and she says that he didn't physically force her but did apply a bit of emotional pressure, you're my girlfriend, everyone does it kind of pressure. So I have that to think about on top of everything else.
He did make it clear to dd that it is "abnormal" for a 35 year old man to have such an interest in a 16 year old and that he will be speaking to him about this. Dd though is smitten, and I don't think she will listen to reason, no matter what.
She has her last GCSE exam on Friday so I think I'll leave things as they are until she's through that, and then try speaking to her again about the whole thing.