Desperately,
Firstly: You need emotional support. Please call or email these people-City Pregnancy- they give free of charge counselling to women struggling with pregnancy loss and are very experienced in this area. It would mean travelling into the City to see them but I have personal experience of them and can not speak highly enough of their service. I believe they would help you.
Secondly: the reason why I asked if any of the pregnancies themselves have been tested (as well as you and your dh) is to figure out where to advise you to seek help medically. Depending on your results, different places and specialists might be appropriate. There are different issues that can be explored and different hospitals are better for different issues. St Mary's for example, are great for some things but not so great for others. People speak very highly of certain hospitals but a lot depends on getting you to the right one for you and your likely problems.
I am not surprised after so many devastating MC you have kind of lost track of what they have/haven't tested for.
It is good your GP is sympathetic and wants to help. I would ask him to help you understand what tests you have/haven't had by going through his notes with you and making a comprehensive list (usually a consultant would copy in results to gp). If he doesn't have enough information I would ask to see the same consultant again for the purpose of getting clarity on this issue.
You would then be in a better position to know where to go to get more answers, and the ladies on the recurrent miscarriage testing thread would be able to advise you well.
I have to say your age is a real warning bell to me: To lose 12 pregnancies randomly would be incredibly unusual but I suppose is vaguely possible if you were say 43/44 and you were coming up against declining egg quality. (Statistically the Miscarriage rate at age 40 is 1 in 2 pregnancies because the egg quality does decline). To lose 12 at age 28 and argue it is random is pretty preposterous.
About this pregnancy: to be honest if you got a positive test on 23rd December, even if it was an early test (say at cd 25) your dates do not make a lot of sense and that does suggest all is not well. It might have been possible for things to be very behind if you ovulated incredibly late in your cycle, but had you done so, you wouldn't have got a positive on that date, so it does seem there might be a problem.
Warning: this paragraph is going to be graphic so skip it if you don't feel up to it. If the worst happens and you go on to lose this pregnancy, I want to give you a chance of getting it tested- if you know if this pregnancy was genetically viable or not it can help you decide where to go in future. I know you have had MMC before, so don't know if you have had ERPC's or if you have gone on to MC naturally. But here is the awful bit: If you start to MC naturally and want them to test, you need to keep the clots that come out so that the EPU can send them away for testing. I'm sorry, there is no delicate way of saying that. What I do is put a sieve across the toilet bowl so that all the bits are saved rather than falling into the loo. It is absolutely ghastly to have to do this, there are no words for the horror of it, but personally I feel for me the horror of losing more babies is worse, if that makes sense, so I do it. Sorry if it is too much detail, and of course I know you haven't lost this pregnancy so there is still hope at the moment, I just wanted you to be prepared if the worst happens.
I would also say that St Thomas' EPU in central London are by far and away the best place I've been to as they really understand the trauma of it all....most places seem not to understand that because its all in a days work for them, it should be for you, if that makes sense.
If you can't cope any more, I would see no reason for you not to go for a day out in London tomorrow to distract yourself and feel so worried during your day trip that you present yourself at their EPU. If you tell them you are bleeding and frightened they would see you.
Sorry this is so long! 