Just wanted to update for anyone in future searching threads due to a similar situation.
The second lot of bleeding I experienced got rid of the retained tissue so that my womb was empty. However, there was still a mass in my Fallopian tube and pregnancy tests were still positive. My consultant was reluctant to believe that it was a heterotopic pregnancy (one in the womb and one in the tube) as the chances of this are 1 in 30,000. They were more inclined to believe that the mass was a ruptured corpus luteal cyst rather than a twin ectopic. They also said I wasn't in enough pain for it to be an ectopic pregnancy, despite me telling them that I have a high pain threshold and give birth naturally with next to no pain relief required.
They started monitoring the hcg levels in my blood every 48 hours. The hormone was decreasing, but not as quickly as they would expect, and still very high for someone who's baby was discovered to have no heartbeat five weeks earlier (the levels were 4100 on day one, 3100 on day three and 2800 on day five). They tried to fob me off with "come back in a week for another blood test" but I had been going through this for seven weeks and had had enough. I insisted they book me to see my consultant the following day which they did.
My consultant scanned me and decided it was a twin ectopic pregnancy and told me that surgery was the best option. It had been five weeks since they had first seen the mass and so there was no way it was going to go on it's own. She was reluctant to give me the methotrexate injection as the pregnancy would then grow before it decreased so there was a serious risk of rupture. I was told to come back at 8am the next morning for surgery. So there I was, on the date of my 12 week scan (and my mum's birthday, who died the year before) heading down to theatre at 8.30am.
The trust where I live have only been doing open surgery due to Covid-19 but I was very fortunate that my consultant had been trained on a new keyhole technique and came out of a clinic specifically to do my surgery. I was the first person to have keyhole surgery at that hospital for three months. As soon as they got inside me, without even touching the ectopic pregnancy, it ruptured. I suffered extensive internal bleeding but they managed to get it under control and they removed the entire Fallopian tube as it was so severely damaged. The consultant told me afterwards that the ectopic pregnancy was so large that they couldn't understand how it hadn't ruptured prior to my surgery.
Anyway, that's my story of how my missed miscarriage lasted 52 days from the first day of spotting: the discovery of twin one having no heartbeat on day 16, the confirmation that they had left the womb on day 33, the retained tissue passing by day 44 and twin two (ectopic) being removed on day 52. I hope this thread helps someone in future.