Go back to her, say you understand it can't be the dog because it was in the livingroom.
Tell her that she must have a faulty letterbox, and its her responsblility, to ensure that nobody can be hurt on her property.
Explain that you have been advised not to work for 6 weeks, and because of this you will be losing £1000 per week in lost income.
Then ask for her insurance details, so you can make a claim for lost income plus pain and suffering.