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