Okay. Lots of information to process here but I'll give you an update - I have read all replies, thank you very much to those who have been supportive and expat please ignore the rude comments directed at you.
Last night the meal went as well as can be expected, I suppose. She was very tense at first but I didn't talk about the pregnancy until well after we'd gotten our food. She'd relaxed by then and I brought out the 'Okay, if you're going to keep this baby, and live under our roof, then what are YOU going to do to make sure it is clean, healthy, and taken care of while you're at school? What are YOU going to do about paying for its nappies, clothes, and childcare as you know we can't pay for it all.'
She blanked me for a minute and said 'Um, I will get a part-time job but then I will have to have childcare and I probably can' pay all the childcare.' I told her there is some help toward childcare costs that we could get. She then said she wouldn't be able to buy things like a cot, pushchair and I said we would help with those as we would if she were having a baby at 30. She asked if I would help her in the night or on weekends and I said no. Not in the nights, ever. And on weekends only if it didn't impact the boys/my plans and we were happy to stay in.
I asked her if she had thought what the baby might mean for the boys/me/her dad and she said 'I think it will be fun for everybody to play with it and love another child, it can stay in my room so won't take up much space and I promise to wake up with it every night and feed it and change it.'
I asked her if she'd thought any more about the other options and she was still very steadfast and said she wanted to keep the baby. I asked if it had anything to do with her Mum leaving her and she started to cry saying she hadn't meant to get pregnant on purpose but she didn't use protection 
so she knew it might happen. She said she knows that we love her and her brothers love her but it's not the same as loving someone 100% and she wants that for herself.
And breathe. We went home and watched a film, had some ice cream and did our own things before bed. DH came home pissed at 1am apologising and falling over himself, I directed him to the sofa. He left for work as normaland apologised again saying we'd chat tonight when he gets in. DSD is having a friend over so she can be occupied while DH and I talk about everything.
Appointment today at GP - GP repeatedly asked her when her last period was, all she could say was 'awhile ago, I can't remember', he asked if she wasn't sure of the date she got pregnant, she got embarrassed and asked me to leave the room. Which I did. I didn't go back in until she was coming out and GP gave us scan appt info (tomorrow morning) and some leaflets about teen pregnancy, and a few organisations we can speak to who can help.
DSD had obviously been crying but I didn't ask her about the appt. I guess more info to come after scan tomorrow and then maybe she'll want to talk about the boy, when she got pregnant, etc.
So we're still a basketful of emotions here but reading through the thread has not only occupied me, it's made me feel a lot more grounded and relatable to other people, if that makes sense. I still haven't told any RL friends as I think I'd feel very judged and have DSD be judged which we don't want. Please keep your posts coming they honestly hand on heart are helping.