There seems to be a flaw in the way Scrivener compiles PDF scripts for the BBC Radio Script format.
The BBC’s guideline PDF stipulates that you should “split dialogue between pages only if at least two lines appear on the first page, and only after a sentence.”
Scrivener doesn’t follow these rules when you compile a radio script from its “BBC Radio Script format”. Sometimes only one line is left at the end of a page, and whole sentences can end up being split between pages.
The only way I could find around this was to export in RTF format, open that in Nisus Writer Pro, use Nisus to make sure that widows and orphans are excluded, and then export to PDF from there.