Woke up this morning to find a d1 error - condensate pipe/trap blocked on my Vokera. Frozen, obviously - except it couldn't be, because it routes to an internal waste stack. I extracted the pipe and trap from inside the boiler and found the pipe blocked with a load of black gritty gunk (which apparently is due to the condensate dissolving the aluminium heat exchanger?). Cleaned that out, still didn't work. Faffed around a bit, and it started working, presumably because some condensate higher up had finally cleared the sensor. Put it back together, working boiler :)
So ice isn't the only problem. Question is, is this level of blockage normal? Or is my heat exchanger on its last legs?