Hi,
Well, can I suggest you point out any corrections to the National Library of Scotland. There is a contact page. There are always going to be issues. As you say move the date by 5 years and the boundary may have moved. You can use the 1900 OS map to see where the boundary was earlier than the drawn 1950’s boundaries shown.
I think it is a pretty remarkable effort given that the NLS would be hard put to justify spending money documenting anything south of the border. Other georeferenced maps are available, just not with parish boundaries highlighted.
I just appreciate how much easier all this is than 40 years ago when you had to buy a physical map and mark it up yourself, or go in to a library, photocopy the map and stick it together.
I am a little confused by your comment about Dinnington, when I use it I am taken to 3 different places, where are the details you are referring to?