Had the same problem today on our Bosch washing machine, model WFF 4010/01. (The number is located on the back face of the door). It's not a big problem, just a broken knob. You will need to get a new knob facia panel in my case part No. AC265574, cost £20.00.
First unplug the mains lead. Make sure that the temperature knob is pointing verticaly Remove the machine cover and the powder drawer. There are three screws around the drawer opening that need to be removed. If you look inside the cabinet and behind the main panel you will see three small plastic retaining pegs along it's top edge and one down the R/H edge. Prise the panel away at the pegs with a flat screwdriver. The temperature selector is clipped to the panel and needs to be unclipped. The panel then comes away. The knob facia panel is held into the main panel with plastic retaining clips and can be removed easily taking care not to break them. Clip in the new knob facia panel. Set the control knob to between 1 and 13 and notice that there is a white plastic universal joint on the back that slides on to the selector spindle. It will only slide onto the spindle at one postion, the old universal joint is still on the spindle and you can use that to turn the spindle so that the two universal joints line up exactly (make sure that you are not 180 degees out of alignment). Remove the old universal joint and clip the temperature switch back into the panel. Now carfully line up the temperature and control knobs with their spindles and push the whole front panel home. Replace the three screws around the powder drawer and replace the lid. Job done.
Dave
March 2007