No joy yet, but here's an update.
Currently, when water initially enters the drum and reaches normal level, the rinse light flashes immediately. The drum does not alternate direction in a wash action before the rinse light starts to flash. I could have sworn that when I first looked at it, there was a short wash period ....
Data below from measurements on our W828.
The heater resistance measured 24 ohms (Fluke 8050A digimeter). This correlates with a heater power of circa 2 Kilowatts (which is about right). Checked resistance to earth (no Megger available) - Fluke measures open circuit between heater element to earth. Found cct. diagram of the W828 adjacent to the drain cavity. I'm puzzled by a couple of the symbols used - haven't seen them previously, not sure what device is represented.
Good design practice would ensure the logic board continuously monitors the water temperature sensor, otherwise a failed heater relay could boil the water in the drum.
I was surprised to find the main drum motor is a DC motor. A miniature alternator is mounted on the end of its shaft, and the output produced is fed back to the logic/control board, where it is measured, so that the logic board can calculate how fast the drum motor is spinning. During internet searches, I found a reference somewhere or other, to a different Miele model activating the flashing rinse light when that feedback voltage was missing.
I connected a variable DC power supply to terminals 3 and 4 on the drum motor, slowly raised the external supply up to 12 to 13 volts. The drum began to rotate, reaching about 2 revs per second at the 12/13 volts point (current drawn roughly 3 Amps). With the motor drawing 3A D.C., the alternator output (terminals 1 and 2) was approx 3.25 volts (AC Volts).
The motor and feedback "micro alternator" seem to be working OK, but am now out of time. Next will be search for possible fuse, check of relays in the motor supply path (not sure how easy they will be to access yet) and the 20 Volts itself. Unfortunately, the schematic of the logic control board is not included inside the machine (and PLC's aren't my strong point anyway), so if the problem turns out to be on the logic board, it will be costly.
Edited for relevancy.