Just to close the thread. The symptoms were a diverter actuator/stepper motor not changing position even tho it would buzz on startup. It was the main PCB that was causing the issue. No visible defects to it just malfunctioning.
Replacement can be had from Ebay at around £80 under the part...
Its designed to create a vacuum drawing air through the flue. I would presume that this is in aid of a sealed room design, to reclaim heat from the exhaust and prevent back flow of gasses (tho with fan over run this should be negligible). If this is against forum rules I apologise I am happy to...
Hi, new member to this forum. Its always been a good source of info.
I had a leaky diverter on a Vaillant 831, and the hot water had stopped working while the CH was fine. So no problem change the diverter, leak is fixed but still no hot water...
Checked a few other bits, the HW heat...