Regarding accuracy, if someones dumbphone is seen to be somewhere near waterloo station for a half hour or so every thursday evening, then a walk round that station, visiting the coffee shops etc, will pretty quickly get you to locate the person.
Same with say a motorway service station....i used to visit a motorway service station straight after work on wednesdays.......i guess they would have noted i was kind of around there and guessed that i was at that service station (eg rather than in the fields around the service station) ...it then doesnt take them long to walk round the services and find someone....so it doesnt need to be terribly accurate to enable someone to find somoeone. So a dumbphone can easily be used to track someone.
A dumbphone tracker can track someone to the nearest phone mast, and phone masts are very close together in london...so a dumbphone tracker can be pretty well accurate. If someone you know with a dumbphone is sat in starbucks in Liverpool street station for an hour, you will quickly find them with a dumbphone tracker.