So you are referring to the hot water tank. How you heat it is still a relevant question.
Are your radiators heating ok ?
Losing heat it 2 hours like that suggests you are using it somewhere. After all you say the water does heat up. If you have a timer, when is it set to come on & off ?
Maybe it isn't on long enough for what you use. Or maybe the temperature it is set too low a heat and doesn't get as hot as you believe.
If you are completely 'at sea' with all this why not try a plumber who has gas pipe training ? They'll boast of being Gas Safe registered.