Michael Holdsworth's project morphed into a study of adult survival and return-rates, following the fortunes of individual birds.
There's a 2020 project report at: http://cambridgeshirebirdringing.org/spofl-finding2/ This has a link to the 2016-2019 Geolocator project.
Lots of new stuff on adult survival; ringing; numbers; distribution; infidelity and bad behaviour.
If we find them difficult to find, how hard is it for them to find each other?