CBD I appreciate it might be difficult to get to the GP on a practical level, but I suspect that you are making excuses (maybe not consciously) for reasons why you can't get to the GP. If you were physically ill and needed to see a GP I'm sure you would find a way. When you say your family would want to know what was wrong, don't they realise already that something is wrong, or are you so good at hiding it. You said in your OP that you can't turn to family/friends - why is this - is it because you don't want to admit that you might have PND. It's a great pity that there is still such a stigma about mental illness that stops so many people getting the help they need.
As for the courts, no one can give you an absolute assurance, because until the case is heard and the judge has made the decision, then no one knows. I have been a soc wrk for 30 years (now retired) and worked independantly for the last 5 years of my career, and did many private law cases, which is what yours would be. You talk of a potential legal battle with your ex. Just so you know it's not called custody any more. If parents can't agree about the children they go to the Family Court and a social worker for the courts investigates the matter and makes a recommendation on which parent should be granted a Residence Order (meaning this is where the child has their permanent home) and the other parent is almost always granted contact, defined by the court if parents can't agree. Neither parent loses parental responsibility and this means that both parents should be involved in any major issues concerning the children.
I am assuming you are in agreement at present. Do you think that there could be a potential battle, as in your ex wanting the children to have his home with him........most men don't want that and are happy with contact. If this is the case I advise you to stick to that - going through the Family Courts is a very stressful, time consuming business. Dependent on circumstances you will get legal aid for legal representation in court, but this is ending in April 2013 so anyone who can't pay will have to represent themselves in court, not an easy matter.
I think so long as the children are safe and well in your care, you have nothing to worry about, and I think you need to see a GP to ensure that if your ex ever does stage some battle, you are seen to be coping, and if your don't get meds for the illness, then there could be a chance that you would be seen as not coping. I think you have far more to lose by not seeing a GP for help.
SO I would stop worrying about courts and get yourself to the GP. I think you should be telling your family and friends you are struggling at the moment. PND is a well known illness and there have been many many women suffering from this illness. You can't delay it because you are bfing because there are ADs that you can take that are safe.
SO get on the phone and make an appt and come back and tell us how you got on. It's sometimes useful to make a list of your symptoms (leaving nothing out) and give it to your GP if you feel you will struggle to say what is the matter.
You won't regret this, the only thing you will regret is not doing it sooner!