The early Combi heaters that use the twistable dials have whats called a flash code to indicate a specific problem with the heater should one occur.
The flash code can be found on the main PCB of the heater located under the removable panel on top of the main heater body.
There will be a green LED lit to indicate the heater has power and next to that is a red LED which will flash in a sequence consisting of short (S) and long (L) flashes.
EG, S,S,L,S,L,L,L,S is number 46 on the code list.
With the information on the flash code the correct measures can be taken to fix the problem.