EBearhug
I googled, but I failed to find when the change was. Mildly surprised, ad proposed changes to TA scheduling tends to warn of the end of the world, or at least letters to Feedback. I suspect it will be in one of the many Archers books, but I can't be bothered to look.
I think you must be right about the change in episode-length happening at the same time, because I can't see how else they could fit the full week into the omnibus on Sunday.
I know from Lowfield that it was at Easter 1998 that they gave us a Sunday episode for the first time, but looking in the BBC Genome doesn't help about how long the episodes were before that, because that just says The Archers started at 19:00 and 14:00 each day before and after that date, which simply Is Not True now so I can't tell whether it was then either. The next programme is said to start 15 minutes later in every case as far as I can tell.