Thank you for the replies, and especially for the clean up!
It is a postcard but is fairly plain on the back - if you are looking at it in landscape format, it says "POST CARD" at the top, there is a thin line going down the middle and on the left it has "CORRESPONDENCE" and on the right "ADDRESS". It doesn't look as though anything was ever written on it.
Going on the dating and age range given it's definitely not my great great grandmother but most likely my great grandmother Edith Mabel who was born in 1880.
Regards, Cathryn