For anyone who is interested, I may have found another possibly bettter solution - well it worked for me any rate and is more in the spirit of what the Apple meisters are after - i.e. proof that the app works correctly on all iPhone models.
I discovered AWS Device Farm:
https://aws.amazon.com/device-farm/
The only real hassle is setting up an AWS account if you don't have one.
Cost is USD0.17 / minute - about USD10 / hr -
BUT you get the first 1000 mins (16 hours) FREE
It is incredibly easy to use - you could even do realistic app testing - e.g. you get complaints about your app not working on a specific device that you naturally don't have.
For iOS you just take the ipa out of ...\Objects\Archive.zip and paste it onto the device web page - 20 secs later you are running.
Images are quite small but a bit of Paint.Net fixes that.
It is not confined to iPhones - has heaps of Android devices also - so testing/troubleshooting on obscure devices becomes realistic.