Come late 2017, and I realised that I needed to work on these skills, so ressurected this project and quickly made progress. The flag finder, flag display and countdown-display were all coded up and worked well within a week of design and coding. This was all using the new TfL Unified API.
The Unified API though is a pain. It's easy to use. Too easy. It returns loads of extraneous information, consuming time and bandwidth. The old Countdown interface is still running well though, and TfL have stated that they don't intend to stop it. The application as it stands makes appropriate use of both APIs, and also a list of bus stops provided by TfL (for the "One Bus" view) which Tfl might stop maintaining.
The app will on start up show you a list of bus stops near you. Clicking on the route number(s) of those routes/directions of interest will show you a list of buses approaching the highlighted stops. Clicking on a bus registration number will show you the details of that particular bus; where it is now, and where/when it's going. Timing predictions are limited currently to 30 minutes in the future though, as that's what TfL tell us.