I have fixed the main original fault which was that the cistern valve was not cutting off and overnight the level would rise until it hit the overflow. The overflow avoided inconveniencing any spiders in the exterior pipe by dumping this water on the floor drip by drip. Resealing the loose joint properly has fixed this big leak - but...
The toilet cistern still appears to have another strange fault where the base is slightly porous and unglazed leading to a slow drip. The glaze does not extend to the underside of the thing. I cannot see any hairline cracks or obvious defects.
My next step will be to put newspaper underneath overnight and watch to see where the latest slow drips are now coming from.
It is also possible that condensation from the steamy bathroom onto the cold cistern surface and then dripping down. If it is a glaze defect any suggestions what to add to the tank prevent it leaking?
I have in mind something like sodium silicate, Fernox or PVA. My dad has clearly painted the (leaking?) underside with ahem something. It doesn't appear to have made any difference though.
I am pretty sure that something else is still wrong with a very slow continuous leak still present but as yet cannot figure out what it is...
Any thoughs for a better way to diagnose or how to proceed?
Thanks, Martin Brown