Drop down menu, force vertical rather than default horizontal?

On my primary menu I have one drop down that lists several other pages/children and due to the length of the pages names they all convert to a vertical view. However, the next menu only has two pages/children and relativley short names and they default to a horizontal listing.

I prefer they all default to vertical. Is there an option to force this? If so I’ve missed it. Or will it require some coding?