So the shop called me.

They had a chance to check it out.
It is a leaking water pump/rings etc and someone added some non-BMW (green) coolant into the system before I owned it.
So they are going to replace the water pump, thermostat and some rings/pipes etc and I asked them to change the oil/fiilter (which I bought perviously and is sitting on the back floor) while they're at it, said he'll do it for $50 which is fine by me.
Total after 1.5hrs diagnostic / parts / 2.6hrs labour / oil change will be just shy of $1100.
Hopefully that fixes it