Update: It may in fact not be Stream's fault at all but rather a bizarre "combined effect" of also setting up a service with Apple (there are some disadvantages to the multi-cloud lifestyle)
So I'm getting all these emails at an .onmicrosoft.com address instead of my usual @dret.co.uk one - mildly annoying but everything seemed to be working - I just assumed it was something I'd done. I was also finding my email seemed to be auto-cc ing me into messages I sent, which I hate.
Finally had some time to look:
Now I have people who will help me and no doubt sort it out - when we do I'll post it here, but it is a VERY bizarre and really annoying glitch. What worries me is that Stream had the authority to muck about with something as basic as my identity online!