Cheers for the quick reply!
Sorry for not being clearer - yes to all questions! - It's when I have the message list to the left whilst i'm viewing a message on the right. If I click the bin/trash icon on the message, the screen switches back to the one-panel message list, as expected. But if I swipe to delete in message list, then the next message is opened.
It's easier to quickly delete a bunch of messages swiping in the left hand panel, but if there are messages I don't have time to deal with, I leave them marked as "newmail", so having them opened automatically is awkward, and I assume the reason for the original option to "on delete, return to message list rather than opening next message" in the first place!
I hope this is clearer. It's been a long day and I'm not explaining myself too clearly this evening!
cheers!