For anyone who has not heard of this amazing facility, just try this.
Go to www.durham.gov.uk
On the home page select "maps" from the cross page index
On new page at end of printing select "Launch GIS now"
A map of County Durham will appear.
With the help of your mouse left button draw a red box round the small area you want to look at. Release the button and the map will update to the area chosen. Keep doing that until the scale in the box along the bottom of the map comes to less than 1: 10000
Then just to the left of the Scale use the drop down menu for "Select Map". You will be able to choose from various maps going back to the OS 1st series of the 1860s.
By using the up arrow you can also access many parts of Northumberland.
Once you are in the area you want click on the cross (5th icon from left along bottom) then you can drag the maps around.
Go on, have a play. I spend hours researching mines this way.