Check that the spindle on the hot water temperature control knob is not broken. To do this, remove the cover for the pcb's, (after turning off the power at the spur, and removing the fuse of course!), if you lower the front control panel down to the horizontal, at eye-level, you will be able to see the spindles for all three control knobs poking through the lower of the two pcb's. Turn the hot water control while watching to see if the spindle turns or not, if not you will have to replace it.
Scott, December 2008