Paul, you are probably already aware of Sarloc which has been designed for MRT’s (Mountain Rescue Teams) locating people in trouble who have with them a smartphone, and for the benefit of other Forum members I will detail this really excellent system.
Firstly, it is not an app, instead it is a webpage developed by Russ Hore, from the Ogwen Valley MRT.
When a person calls for help using a smartphone, they are sent a text message with a link to a webpage. By simply clicking this link, a page opens in the phone's browser, which queries the phone display its location in Lat/Long, both to the caller and simultaneously over the internet to the Mountain Rescue Team's database. The MRT call handler can then see the phone's location displayed on a digital OS map display using an app called MRMap, developed by Dave Binks and Rob Brookes in the Lakes.
The list of smartphones this system works on is impressive, from most iPhones, BlackBerrys, Samsungs and HTCs, plus is growing by the day.