Agree with Ragged.... In NO way should you feel bad for not having noticed...secrecy and deception are all part and parcel of the illness. Beat are fantastic, and their services have helped me over the years. GPs are fine for monitoring weight and referring you/your daughter on to the appropriate practitioners, but in themselves - from my experience - have little training in EDs and can be unsympathetic. If/when you go in, don't expect them to have all the answers for you (they may just tell your daughter to 'eat more', or refer her to a nutitionist...whn I doubt the problem is WHAT to eat, more getting her to eat it!).
Important to remember is that your daughter has developed this for a reason. She may not even know why herself - and it can take years to get to the bottom of causes (sorry to sound bleak, but trying to be realistic). By the time I was 23, I'd had an eating disorder for longer than I hadn't. Only when I stumped out the negative influences in my life did I realise that they (he) were (was) the cause.
Depending on how 'far in' to the illness your daughter is, this could be a long ride for all involved. I pushed my parents beyond limits, and though they may have given up hope on numerous occasions, they never disappeared.
Firstly, it sounds to me very much that you are right in your convictions. You need to confront your daughter with your concerns, so that they are at least no longer speculative. Chances are she will deny...but at least YOU are being honest and open. Thereafter, a tough love approach is, in my experience, the only way. Asas jobbo says, include her in family mealtimes. This is likely to be difficult and a battleground. Don't compromise on what you are having; don't include her in the making. As nice as it may seem for her to be involved, there are so many opportunities for her to 'sabotage', (cutting down on ingredients, not using oil etc), or to begin to negotiate with you so that a downward spiral ensues, and her fixation on food continues (you may think that you would notice, but where eating disorders are concerned, there is always a way of getting 'one past' a person). Don't let her go to the loo, leave the house or leave your company within 45 minutes of finishing a meal. If she ups her intake, the eating disorder mind will look for a way to compensate; vomiting, exercise and laxatives being the common ones (plenty of exercise can be done behind a closed bedroom door, so don't be fooled by that one either!).
In terms of therapy, it'd help to get an initial idea of where this may stem from if you can (but, as I said earlier, this may remain a mystery). I'd advise 1:1 psychotherapy (as opposed to counselling....counsillors cannot advise or treat...they can only ever ask inquisitive questions to encourage a person to think), and family therapy if you have the option, to encourage you to gain an insight into your daughter's mindset, and for your daughter to feel more comfortable in communicating with you.
Get your daughter's school on board. I've done work in schools in the past, implementing supervised lunchtimes whereby a sufferer and a trusted friend sit in a classroom alongside a teacher to eat. The teacher can either just take a monitory role (noting down the food that is consumed/left); or they may need to take a more active role (encouraging your daughter to eat, if this is something she responds to - some do not).
Sorry if this sounds bleak...I really don't mean it to. Eating disorders ARE NOT a diagnosis for life, but for this to be the case, they need to be treated in the right way. Also, your daughter is still young, so a certain amount may well be down to body image. The risks, though, when EDs spiral, are very severe and they can take away years of a person's life (and the lives of their loved ones for that matter). Beat also offer support for family and friends of sufferers, should you find it helpful.
Feel free to ask if there is anything I've missed out, or if you want to pick my brains at all!