Toilets leaking into cistern bowl

Help! I have enlisted the help of a plumber who is also stumped at how to fix this problem.

I have a combi boiler and 4 toilets in my home. There is an ongoing problem which comes and goes where when you flush the toilet, that toilet and the others will all start to overfill, and will just leak continuously into the cistern bowl. It appears to be pressure related, as when I turn the stopcock right down, the leaking is minimal, however then I have no pressure in the taps, shower etc.

Plumber suggested pressure regulator but will obviously will mean my taps will practically dribble out permanently!

Plumber now looking at the toilets themselves, and looking at whether it is the fluidmaster diaphram wasger that needs replacing. Currently black and believe that is standard, unsure if they do one for high-pressure??

Forums on toilet overfilling suggest fixes to the toilet, but as at least three of my toilets do this, wonder whether it is a system problem??

Would really appreciate it if anybody has any suggestions?

Are all the toilets cisterns identical? It sound almost like when you flush one and you get a slight drop in pressure the inlets start to leak. If they are identical I would look at a design fault in the mechanism.

Are the toilets filling via the drains which suggests a blockage

Speak to the manufacturer if you have a fancy flush mechanism.

I just responded to a similar thread - the Fluidmaster is great for siphoning. You must use the part that attaches to the overflow tube and do not insert the refill tube into the overflow tube.