I don't know why I hadn't noticed this over the years, but what with all the rain and being forced to stand outside to smoke these days, I looked up to the inside of the main door the other day and noticed that the inner header has a shallow curve for an arch, and this was the original opening before they put the new facade up around 1740.
It looks as though the doorway may have had a slightly rounded top, and because of the angle of the jambs either side of it (before the left one was blocked to form the lobby) the door was split into two as it is now, and lay flush and out of the way against the bevelled jambs when open.
I will drill a little hole into it at some point to see if it is wood or stone.