my Glow worm 30CXI showing fault code F5?

I tried to switch the heating on this morning but it didn't come on. The weather is freezing out, -3. when I checked if the hot water would come on, no water came out of the tap. I've tried heating the pipes up including the condensing pipe outside. The water comes out of the tap now but doesn't heat up and the heating still won't come on. The display is showing F5 fault code but can't see how boiler is overheating as it is not coming on. Boiler was installed in 2007 and does not have an external reset button as display is different to newer models.