The story on iOS is a little different.
In the Google dev console I can see downloads AND current installs. On iOS I can only see downloads so I've no idea how many people are still using using Resus Algorithms.
On iOS there are 4910 downloads. On Android there are actually 7570 downloads. 3000 people uninstalled Resus Algorithms on Android. Can I accept the same proportions on iOS?
I'm going to assume, for arguments sake, that all 4900 iOS downloads represent users.
The latest NHS numbers, from this link - http://www.nhsconfed.org/resources/key-statistics-on-the-nhs - state there are 518864 doctors and nurses in the NHS at the moment.
So I have 1.81% penetration of my potential users!
In July 2014 this was 0.54%. Not bad.