Miele Dishwasher G1040 - Continuous Intake

[Cosmite]

It is staying on its wash cycle (used up a cylinderful of hot water!), and the Start/Stop button has stopped working. (That is, when the power button is pressed, it goes straight into washing.)   I have had the top panel off and the Start/Stop microswitch seems to operate okay.  Now I am wondering if it is a failure in a timer or programming unit, or if it could be something to do with a sensor.

It is an old machine, second-hand, but has worked superbly until now. I would be grateful for any advice on how to pinpoint the fault.  

The entire motherboard.looks easily replaceable so I wonder if they are available anywhere and not too costly.