adding more than just the primary calendar account of a google apps user to iphone/ipad

Private google calendars, which have been shared with select other users:
In IOS, these are added: Settings --> Mail, Contacts, Calendars --> Add Account --> Other --> Add CalDav Account

server= calendar.google.com
username=  your own gmail username (ie your email address)
password= go figure
description= Calendar Description of your choosing

If this is your own primary google-cal,  (default) cal for an account, you're done. (These are referred to by that users email address)

If it's an additional (non-primary) calendar account, of your own or another persons primary/additional:

go to 'advanced settings' section of the calendar account.

Nb. Stupidy in newer ios you can't edit 'advanced settings' til you've created the new calendar, so the only thing I could see to do was save it with your own primary calendar email address, then re-edit and insert the calendar ID of the 3rd primary/secondary calendar ID in the url field:

The google ID is to be found in the googlecal 'calendar details' section of main google apps website.

https://www.google.com/calendar/dav/ [ your Google Calendar ID ] /user

This used to look fine in older ios but in newer (ipad?) it creates another level of calendar account which shows in the list. Confusingly when you go to advanced settings now it doesn't show the ID url as older ios used to - BUT they do work!

Public 'view only' cals ('subscribed' calendars)
Settings --> Mail, Contacts, Calendars --> Add Account --> Other -->Add subscribed calendar
(you'll need to cut and paste in the url: they end in .ics)

BTW you don't need to do any of this in Android!