Thanks winni. This is getting off-topic, but: the region was already set to British English so I added a second language and selected British English. It made no difference. The only way that I've found to correct it is to revisit system settings -> input devices -> keyboard and hardware layout -> layouts, and then unclick and re-click the configure layouts with gb english selected. It shouldn't be necessary -- Plasma shouldn't be being so bloody minded -- which is why I think it's something to raise on a KDE forum rather than this forum. But thanks for trying to help.