Accidental mould inhibition

If you used one of the cheaper antifreezes which has a high concentration of methanol then that would probably be the explanation. Methanol isn't as good as ethanol or isopropanol as a disinfectant, but it will probably deal with moulds quite well if left in contact with them for several days, even as a dilute solution.

Grew mould where? Never seen this before.

For once Harry, I have to agree with you!

The mould grew at the water air interface and spread to the underside of the toilet rim, presumably by splash contamination. I've not seen it before, it's only one toilet of 4, and purging the tank and bowl over a couple of years failed to deter it. The anti freeze was Poundland and one litre? was used per toilet, after draining down.

Any mould in the cistern when you added the a/f? Otherwise I would have expected pretty clean water entering the pan after the flush and be unlikely to support mould growth.

If the cistern was supporting mould, each flush would dilute the content by 4 or 5 to 1 and might not be enough to kill it. The antifreeze has possibly done you the favour.

If it comes back then one of those disinfectant blocks in the cistern might be a solution?

I've found it cleans the drive (at least, small patches of it) nicely, too :-)

Both toilet blocks and bleach in the cistern had been tried. A block and bleach in the pan was also tried. This toilet was in use daily, yet still grew black mould. No mould was apparently present in the cistern, just iron staining which comes from our water supply.

A real mystery indeed!