[Stooky Bill]

What is network in this context? Since all local stations are their own entity, not part-time services opting out of a sustaining feed like on television.

Who would have heard the 18:00 bulletin that seems to have gone out across England on a normal day?

They have a service called CNS (Central News Service) which provides national news clips and bulletins for the local and nations stations. I assume it was that. I'm not sure if the bulletins are put out every hour throughout the day but most stations only seem to use it in the evenings and weekends

It replaced an older long established service called GNS a few years ago. CNS is based in Salford whereas GNS was came out of London.

During covid to save on staffing there was another generic BBC News bulletin that Radio 2 and 5 Live used amongst others. I think Radio 4 might have at some times too