It's on the tools menu from the menu bar. click alt key if menu bar not shown

Thank you so much David, I thought I was going mad with this. I'd not fired it up for a few days, and was going to the burger menu as the main menu wasn't showing, and it's not on there. I hadn't realised that the menu bar still existed and is optional.

I checked settings and can't find an option to enable the menu bar permanently. Is that possible? I use Alt-Drag on Windows, so the Alt key isn't available for application use unless setting up an exception in Alt-Drag, which is awkward. Alternatively, a way to choose what menu options are on the cut-down burger menu would be nice.

click view>toolbars and tick menu bar

Thanks again. Maybe an update had reset the view.

I think there's scope to make some beneficial changes to the upper region. I get that hiding the menu bar saves space, although I'd rather that the search box and burger were gone and there was just the standard menu, which would actually reclaim more pixels. Search could be to the right of the menu bar, or it could be in the empty region at the very bottom of the main window. Alternatively, the search field could be hidden until shown with ctrl-K. Customising the mail view does allow the search box to be removed, but I couldn't find a way to get rid of the burger, so there's still dead space above the menu if search is removed. The burger could be made removable, or it could relocate to the right side of the menu bar when there's nothing else in the area above the menu bar.

I agree that the decision to hide the menu bar, which began back with release of version 78, may have been ill-thought. Its absence is the cause of problems posted daily here. I have submitted a request to restore it, but I doubt that happens.