New Storage Thermostat?

Do I need to replace the storage thermostat on my boilermate 2? We leave the heating room thermostat turned down during the day when no-one is at home. When returnig home if the hot tap is turned on the water only reaches lukewarm. If we then turn the heating on the radiators only become lukewarm but slowly warm up to their operating temp, as does the hot water in the taps. This seems to point to the storage temp in the tank, so is a thermostat change required?

If you think the store stat is faulty, you can test it.
Or what temp is the unit heating up to?
But it would be better if you had a good read through the BM II posts.
First thing with any BM II is to check the water level in the F& E tank.
Then check unit will take water.
AS yours could be partially empty.
All in those posts
Mike