This happened on our WD860, and is a common problem. The fault is the temp sensor, which is not correctly rated, so replacing it just creates the same fault again. Our Hotpoint Engineer replaced the sensor for the correct, higher rated one, and it worked fine.
So a) It is the manufacturers fault
b) the thermostat will not reset itself; it broke.
However, the machine has just developed another fault, where it will only do a test cycle, and not wash, and the screen display is failing. Not too good! Get another model under warantee?
Mark, July 2006