RE: Purity CC WC suite - Cistern with dual flush valve CP button?

Purity CC cistern

For some months previously:
After flushing, water seeped slowly into the toilet bowl for a while. Afer a while, the water would stop seeping into the bowl, and the cistern flushed as normal.

Now:
More water is seeping into the toilet bowl and this is continuously; which is probably why the cistern does not fill so it is not possible to flush the toilet.

Could you please if possible diagnose what the problem might be; and please point us to:
a) which spare part we should buy (we feel we should change the entire black metal & plastic inside fitting of the cistern)

b) a video showing how this can be replaced

c) any plumbers in our area (Richmond TW10) with ideal Standard expertise.