Mrs. Drover is the main character of the story. Apparently she flees from London and moved to the countryside but in the story she is coming back to get some personal things for her family that were left behind, so she is back in London. There are many reasons why we can say that
Mrs. Drover is mentally unstable. First of all, she encounters the house in a bad state, "dead air went out to meet her”. There is an explanation for this, the bombing left the house in a really bad shape with cracks on the walls and the dead air comes from the house being abandoned. That's what happens to abandon infrastructure the air is trapped.
Furthermore, we learned that her fiancée was presumed dead long time ago. The transition of coming from a relaxed countryside home to the bombed city might have created flashbacks about her dead fiancée. And made her to think about him. This caused her to write the letter and assume her dead lover wrote it. She is clearly unstable " she could not remember his face” this is because of the fact that she lived a traumatic experience she cannot think about the past.
And lastly the screaming and hitting of the glass on the taxi is clearly she had a mental breakdown. All the flashbacks and the letter created this to happen, all her repressed memories came out in the breakdown. I can't defend the point of the taxi driver just driving but I can hardly find where the ghostly story is.
In conclusion, Elizabeth Bowen wrote this story in way you can interpret it according to your beliefs. You may find facts that will make you think that Mrs. Drover is just crazy or you can either find the spiritual meaning of it. Either way it's a good story to read with interesting plot.