Hi Stuart,
Couple of questions,
When you have a csv on your iphone can you open it natively without having numbers open first? my understanding was that numbers needs to import a csv into its own format. ?So doesn't actually have csv registered as a file extension / format within the app itself, pretty sure that csv needs to be registered within the app when it its compiled to open natively.
maybe a way around it is to setup a permanent file download folder for your browser, then at least you would know where it has been saved to.
I don't have an iphone, so may be steering you wrong, google says ;-) the following
https://www.google.co.nz/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiAgPanpLrXAhVFFZQKHZogClUQFgglMAA&url=https%3A%2F%2Fstackoverflow.com%2Fquestions%2F22967013%2Fhow-to-associate-csv-file-to-my-app-in-ios-7&usg=AOvVaw29cRfIoQaM5JgNj8QZIDRX
That maybe explains what's happening at least.
cheers