Hi Stuart!
On the Wikimedia page you suggest adding drinking_water=yes to a drinking fountain (amenity=drinking_water). However, this is redundant as amenity=drinking_water already implies drinking_water=yes. [1]
[1]: https://wiki.openstreetmap.org/wiki/Tag:amenity%3Ddrinking_water
By the way, you may also be interested in mapping amenity=fountain, i.e. decorative fountains, [2] as they often provide drinkable water too. Note that for amenity=fountain it makes sense to add drinking_water=yes/no.
[2] https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dfountain
Regards
Markus