Toilet cistern overflowing when shower turned on. (Is Builderrsmate there please?)

[MichaelZZ]

This might sound highly unlikely but my daughter has reported that her toilet cistern overflows when she turns her shower on. (The toilet has an internal overflow so no danger of a flood.)
She also says that if she flushes the WC there then doesn't seem to be enough pressure to have a decent shower.
In addition, although she has had the problem intermittently for some time, yesterday when she ran the wash basin tap it effectively flushed the toilet.
Her initial thought is that the problem is something to do with the cistern and I suggested that she should take the top off to see exactly what's happening when she runs the shower. (I'm hoping that she'll be able to take a short movie clip to send me as she's in London and I'm in Yorkshire.)
Is it possible that there is a pressure problem somewhere in the system that would account for the problem or would replacing the "innnards" of the cistern be the answer? Or is it possible that the pump is nacked?

[HowardKennitby]

Do you mean that the toilet bowl fills with water and then overflows?

[DaisyNonna]

New toilet put in by my land lady when I was away. No idea how to take the top off the cistern. Asked the plumber when he came to fix my taps. He had no idea either. If it ever floods the downstairs flat at least I tried.

[Hopkirk]

Sounds like a useless plumber.

[douglas9401]

DaisyNonna, if your new cistern has a dual flush with two buttons inside a chrome ring then the ring may be securing the lid of the cistern. Ours does.

[DaisyNonna]

Sorry, no dual flush. All I want to do is to be able to take the top off the cistern, to put some cleaning agent into it. Loo itself is squarish shape. Difficult to clean.

[MichaelZZ]

Please see new post above.