Found the button. After reading que "Do I need to replace the NTC in the heater of my Bosch WTL5500?" posted Nov 2005, went in hunt of NTC. Take back oval cover off (note - not all the screws are the same and a couple of the bottom ones are likely to have some of the sealant on them). You'll see a diamond shaped metal box. On the top/left edge is the NTC and on that is the reset red button. Restored to working order.
awol
January 2009
Had the same problem a year ago. The heating element was fine but a trip had gone. The Bosch man showed me a trip button to reset. Was convinced the button was accessible from top, but now can't find it. About to take the back off. It could of course be the heating coil has gone, but worth trying the button first. If I find it will let you know.