Paul , If your shower has been functioning normally over the summer, the chances are its a combination of the cold weather we are experiencing and maybe partial blockage of the filter. When you turn the control for more heat you are essentially reducing the flow over the electric elements. This is fine during the summer when the rising main supplying your shower is 10 - 12 degrees. but somewhat different to now , which is about 1-3 degrees. To obtain sufficient heat you are probably turning the hot control to max hot and as a result your flow drops to such a low level the unit switches of and runs cold. You describe it as a 7000. does this mean its is 7kw. That could be your problem. Have a competant person check the service filter .
Alan
December 2008