Just wanted to give an update. Over the past few days I have been doing a lot of Internet searching trying to make sense of what is happening to me, and although it wasn't always what I wanted to hear it did help me to understand and prepare myself. The worst was coming across threads without an outcome though so here it is!
Unfortunately yesterday (16 dpo) my line got even fainter. Today (17 dpo) got a bfn and started spotting this afternoon. So that's that. I did also have a bit of a BBT plummet this morning so that was also a sign of things to come.
I know chemicals are common, but given that it followed a MC I'm now worried so that there could be an underlying issue. Any reassuring stories from people who have had similar and gone on to have a successful pregnancy afterwards would be very welcome!
I don't subscribe to the view that the solution to chemical pregnancies is not to test early. My average cycle length is 26 days (has been as short as 22 in the past 5 months) so when I tested at 12 dpo it was already well past my period due date - I'm effectively a week late today (well, not anymore) so I would have known i was pregnant whether I had done a test or not.
I know a lot of people say stop testing as soon as you get your bfp, but for me I'm so glad I continued to psoas like a crazy person as I knew something was up at the earliest opportunity - which for me was a good thing, I never allowed myself to get excited this time. It's a personal thing, but for me ignorance isn't bliss and I'd rather be prepared for what's coming.
Just my opinions 
Thank you for everyone's support and wisdom, it really has helped 