Re: Cistern overflow!
That's correct, if it overflows it goes into the pan.
It seems like the Valve isn't shutting properly, possibly faulty.
I'd screw it down a lot lower than the markers and then try readjusting it.
I'm ok at replacing them but not well up on the mechanics etc.
Gasman or Fender may be a better bet, although they're heating engineers their understanding of all thing plumbing will be far greater than mine.