The heater is a 24 Kw Thermelec heater. As the name suggests it consumes 24 units per hour when the heater is warming the water. Its anatomy includes (1) - Heater Trip (up for ON), (2)-Mains Power light, (3) Mechanical Thermostat, (4) Pump Interlock lamp (lit when pump is running), (5) Heating lamp (Lit when heater is heating), (6) Other service lamps (OK when all are lit), (7) Over temperature reset button (safety cut-out). The power is spread to 6 elements all of which look to be working. Generally these heaters only need servicing when they fail and if an element fails it can be disconnected for a quick fix. Above is the report I had done on the heater in 2023 which can be seen and the heater hasn’t been used since then. In full working order
eBay