open the tank top or cistern cover .Turn off the water supply to it, the problem is the inlet valve is leaking. you will need to open this and replace sealing washer .Can't be more explicit as there are several different types of valves with different types of washer
Ron Bullen
November 2021
Is this your condenser boiler draining? Is it dripping all the time, or is it just dripping when the boiler is on? (Producing heat or hot water.)