I'm really sorry to hear about your loss Laurz, and I agree with how hard it is to move on when your body doesn't seem to have done so.
I agree with what the others say, definitely ask for a scan. Even if there is the tiniest amount of retained product then the bleeding can just keep going. If there is any tissue there, your body will keep sending blood to the area and you could just keep bleeding, without it effectively clearing the uterus. There's also the concern that your iron levels could fall quite low with so much bloodloss.
I know with my last miscarriage, even though the blood loss was huge (it blew my mind how much blood you can lose without passing out or dying) the Drs and nurses told me they don't like to intervene unless absolutely necessary.
With my first miscarriage I chose to do it naturally, which ended up taking months (I was also 12 weeks) but I was monitored quite closely and had ultrasounds to ensure my uterus was clearing, and I took herbs which were effective in helping it to do so.
I hope it sorts out soon for you so you can move on, and have a healthy pregnancy next time