I expect this is obvious but somehow I have ruined my mail setup so that I no longer have the facility to view the messages without opening them. Instead I now simply have a page-long list of messages in my inbox and no split screen. Can anyone tell me how to get it to revert to the default view please.

Sure...

Look at the bottom of the Mail.app window. There's a very small icon of a circle/dot/button in the center. Mouse over that and you will see your cursor become vertical arrows divided by a horizontal line. Drag up, and your preview window is back.

You are the best thank you so much.