I've just spoken to the obstetrician I'd arranged to care for me through my pregnancy and she's putting me on antibiotics as of tomorrow, and referring me for a blood test (also tomorrow) and I'm to organise for the results of the ultrasound (Wednesday afternoon) to be sent through to her. She thinks that the slow drop in the hCG level and the current odour of the clots suggest there could be some retained product.
On the upside, I feel relieved to have spoken to her. So blood test and antibiotics tomorrow! yay. Thanks Sarah and Owl.