Without exact model number and type, precise advice is hard to provide. I'm assuming that your toilet cistern is hidden in the wall and you have a large flush plate with the buttons you press to flush.. If this is the case you will have to remove this flush plate, proabably by sliding upwards and pulling bottom forward and out. This will reveal the workings of the cistern which will be two main components, the flush valve in the middle and the fill valve to one side.
The fill valve will need attention to solve your issue. Replacing it for a new componant is the easiest method, but a repair is normally possible by removing the fill valve, stripping it down and cleaning all the washers with HG Limescale Remover, pay particular attention to the water inlet which will often be restricted limescale deposits. The fill valve must be precisely rebuilt with all the washers in the same position as they were before cleaning. Also make sure that the plastic parts where the washers seat are also perfectly clean. Limesacale remover is an acid, so precautions must be taken to protect your skin and eyes when using. Thourougly rinse acid off before puting back together and be careful not to loose the small parts down the plughole when cleaning!
Rupert, December 2019
