Deeno, I'm so sorry this is happening to you.
I had the same experience a couple of weeks ago (was also about 12 weeks).
Do you have a local Early Pregnancy Unit you could call? I rang the EPU when it started and they said to come in if I felt faint. In the end I collapsed on the kitchen floor after losing quite a bit of blood/clots, so DH had to phone an ambulance.
I think it might be a help to save the clots, horrible though it is, because the hospital can check them to see if everything has come away. I didn't keep mine though (flushed it down the loo), and it wasn't a problem - I just described what I'd seen.
In the end I was glad I went to the EPU because they checked me internally to make sure my cervix was clear (it wasn't, so was swabbed, like a smear test - nice), checked iron levels etc.
Much love to you, I'm sorry you're having to go through this. Please take care of yourself, make sure you take things really easy afterwards - I was surprised and shocked by how much it took out of me, mentally and physically. Take care.x