Thanks to another mumsnetter I have been introduced to an excellent app which can be used to find ... errr ... 'lost' iphones and their owners (should they be lost too).
All you need is the password to the apple-id the phone is running from, then turn on "find my iphone" in the icloud settings and next time you need to know the wherabouts of that phone you can log in to icloud on any laptop and locate the phone. It brings it up on a little map and doesn't notify the phone that it has been searched for.
Just saying.