Maybe PigletJohn is around, or someone else came across this before?
System is on combi boiler, it's a flat and has two bathrooms.
So when the kitchen tap or a shower in the other batroom is open the toilet in the second bathroom self-flushes - but then stops. Also when you just flush it, it takes a while to fill but it does stop rather than leaking (unlike the toilet in the othe bathroom).
Is there a faulty valve or is it a boiler issue? It mainly bother me as the pressure in the second shower is low due to some water going to that toilet at the sanetime, but also obvs wasteful.