The text size in the reading pane has suddenly switched to a very small size without my intervention!
Despite trying to change the size through the settings section, nothing appears to have any effect 
How do I rectify this error and restore the correct text size I selected originally ??? Thanks
That is most likely a zoom setting. Use Ctrl 0 to reset it.
Ctrl + or Ctrl - zoom in and out, or Ctrl mouse wheel does the same.