Hi Trueblue,
It's a long shot, but as you have Bertha's date of birth, she might be searchable in deaths post-1968, as after that the d.o.b was included on the death cert. Provided of course that she did die after 1968, and provided the person registering the death knew the birth date. And providing she kept the name Bertha.
As I say, a long shot, but I have found a great aunt like this - searched the GRO deaths using just the first name and the date of birth. Came up with one that fitted, but with a different surname because she had married.

Barbara