Just been listening to a radio documentary on leaking loos and it is quite sobering. The problemo is the fancy valve flushes - often double flushes. Twofold.

1) fancy arty farty designs means there is no consistency on which is the small or large flush. 50% of people get that wrong!
2) they use valves to release the water which are prey to detritus. Once that is trapped the valve leaks. And there are two of them so double the chances. Yea, Yea - they should be maintained. Yea yea, who goes into the tank and changes rubber that you never have to hand anyway?

The solution is the old syphon mechanism. No valve, no trapped bits. Except the modern plastic chambers therein don't act like the springs they are supposed to, they fatigue and especially on cold weather.

So that 5400 litres is looking even smaller, and rainwater carries detritus. Ain't life just complicated?