Filled up my water tank - Noise gone but only warm water?

My Pulsa Coil A Class has never given me any hassles since my new build was built 3 years ago...until now. I recently started to hear gurgling sounds from the boiler and had not hot water. after a days I read the manual and realised that the top tank should be filled with water which I have now done. The noise has stopped BUT I only have lukewarm water coming out of the taps.

I called in a plumber who recommended I use the boost button which I have never ever used. After an hour the boost button switched itself off and I have hot water! This only happened this morning so I am happy that at least everything works but am unsure if this is the case and if my usual problems will return tonight once I get home.

Does anyone know if all will be ok or if there is anything I need to worry about? I dont plan to use the boost button again.

If the unit heats up during the night then you are ok.
The boost button just heats the top heater.
The difference is when using the boost is;
Limited hot water, although enough.
And dear as full rate not half price electric.
Mike