I diagnosed the toilet issue; the plastic connector piston in the syphon broke, where the metal that hooked into it, connected to the handle.
It’s broken in such a way that the piston has dropped down, inside the body of the syphon, and can’t be retrieved. Gaah!
And this bog has many other issues. One of these was this:
So I fixed it:
The new pipe fits much better. I re-used the old bend-couplings. It seems ok? It’s not leaking as it did before.
But the main issue – the cistern isn’t working – remains unresolved. At least we can flush using a bucket of water. So the bog is usable. Albeit in a labour intensive and annoyingly primitive manner.