Thankyou so much for the replies everyone; although I haven't agreed with everything that's been said it's been very useful to hear other points of view.
Thankyou MokunMokun; I'm really not sure if they've actually "gone all the way" but I'm sure that even if they haven't it won't be far off.
Thankyou again tigermoll; I really am confident that she is no longer doing it as I have seen her naked when I was helping her out of the bath the other night (she has her arm in a pot at the minute due to falling off a wall trying to avoid a group of boys - oh the irony!). I really hope she doesn't feel worthless; I have always tried to bolster her self-esteem - always told her she's beautiful from day one, praised all of her efforts and achievements, etc. She says sometimes that she thinks she is "fat and ugly", and I know that unfortunately this feeling is common in girls of her age (I was the same, as were most of my friends - some still are!)
Thankyou cheesecakemom; teenagers are bloody hard work! I think a lot of the behaviour is attention seeking in nature; I also realise that she was mega pissed off with me and would have been trying to upset me (as they do). I have perhaps been a bit slack with some of the boundaries over recent months due to being so tired with a new baby; things like letting her stay up late with me and not limiting her screen time. All change on that front!
Thankyou MidniteScribbler; you're right, I have referred back a lot to my own teenage years, only because I have never had a teenager before and my own experience and those of my friends are all I really have to relate to on that score. Believe me I am concentrating on my daughter as well though.
Thankyou Purplepoodle; I had a horrendous experience on the implant, with depression and severe anxiety, so I am reluctant to go down that route, though I realise that it is an option to be discussed further.
Thankyou Altinkum; no I haven't told the school, as I'm still not 100% sure how far things have gone. Besides which, I'm not sure what they could do? The kids are both the same age and both 13 so apparently able to "consent". Perhaps I have handled it wrong but for the right reasons, i.e. keeping my daughter safe and trying to prevent a possible teenage pregnancy.
Thankyou Piper; I had thought about the "skipping classes" thing; this would be very out of character for my daughter but it's not beyond the realms of possibility.
Thankyou Eastpoint; this is one of my concerns: it's not just about the (possible) sexual relationship between them; it's all a bit "heavy" on the emotional front as well. Unfortunately my daughter is one of those kids who never sticks at any kind of extra-curricular activity; we've tried to get her into stuff and she hates it all! Academically she is outstanding but has never managed to sustain any kind of hobby! (She was a brownie; think that lasted about 2 months).
Thankyou Doobigetta; you could be right there. As I said upthread, I can't afford to NOT check her phone from time to time after the previous safeguarding / potential grooming issue (which occured the same day that they had a big talk warning them about it at school!). I don't think my daughter would feel at all comfortable talking to her Nanna; she thinks I'm a fossil at 34 so don't think she's listen much to a 70 year old!
Thankyou redexpat; I understand that she has a right to patient confidentiality; I just personally feel that at 13 they are just children and decisions such as whether or not to have an abortion can be too much for them to handle on their own.
Thankyou for your kind words Balloonslayer.
Thankyou Moniker1; I am sorry to hear of your difficult childhood; I didn't have an easy time of it either, which I guess may be why I'm a little overprotective when it comes to my own children. It is really important for me for them to be happy and safe. I really do enjoy my daughter's company; she is great fun and bright as a button, and we share some common interests, like music and films. I'm not disappointed in her; I understand that they have been together for two years and are very close. I'm more just worried that she hasn't got the emotional maturity to cope with a sexual relationship just yet.
Thankyou LumpySpacedPrincess; I agree with your comment about the pornification of society. I think it's fantastic that adults can participate in pretty much any legal sexual activity they like, with who they like, and that as a society we have moved on from prudish and homophobic attitudes. I just wish that our children could be children for a little longer.
Thankyou addictedtobass (great user name; so am I!), I'll have a look for that, just as much for my own benefit as hers.
Thankyou wasitsomethingisaid; I really want to let her live her own life, and I know that as she is growing up I have to watch as she makes mistakes and just be there to help pick up the pieces. But it doesn't matter how much I know this in theory; in practice it is so so difficult to watch someone you love more than anything in the world doing something which you know could end up hurting them.
Thankyou Hairtoday; I have read similar threads on here too where people were being told that they basically needed to step up and stop the behaviour. I'm quite surprised that i haven't had a single response like that; I was expecting some! I'm sure if I was saying that they had only been together for a couple of weeks the responses I received would have been very different. It's good to read that someone understands where I'm coming from with the whole "checking her phone" thing; I really don't feel that this is wrong as it is purely motivated by keeping her safe from grooming, etc., a problem so prevalent and as I said upthread, something which has affected my community with a horrific outcome.
Thankyou NickiFury; it's a good thing I have a thick skin as I have read some things about myself that aren't exactly pleasant! Yes I am really trying to look out for her.
Anyhoo, we had a brief chat tonight (baby got a runny bum bless her so I've been very busy), and I've told her that before I can think of letting her out and about again we need to have a serious talk about everything, just the two of us. I have talked to her about sex before; I have always tried to answer any questions she has had in an honest and age-appropriate way (as when she memorably asked me what a blow job was when she was 10...). However we've never really just sat down in one session and really talked about it in-depth. She told me that the boy's mother had gone through his phone as well but didn't seem too bothered, and that his dad had said "go on son" (how true this is I don't know, but having spoken to his mother about it it's fair to say they're not really that arsed - his mother lost her virginity at 11 so doesn't think it's any big deal!). I got the "sex talk" at about 11; myself and my cousin the same age, my auntie (a nurse) got us slightly tiddly on Tia Maria and coke then got "the book" out! So I'm going to get her a bottle of Babycham (!) or something and just try and discuss it sensibly, "ancient" woman to "almost" woman. I'm not going to make it easy for them in any sense; still only allowed in communal areas and I'm not letting her go out "hanging around" while it's dark nights. I'm also not leaving her with the house to herself for the time being. As I said upthread she's at her dad's the majority of weekends and he lives two towns away.
I really don't want her to feel that sex is a "bad" thing; it's bloody fantastic with the right person and I also want her to value herself enough to say "no" if she wants to. I know I have fucked up in the way I have handled this; I really want her to be able to come and talk to me and I want to respond in a non-judgmental, less emotional way.
Again, thankyou for the replies; much appreciated. I'll keep you all informed!