Thanks @Maus for the link.
Having just tried to follow the advice on TJF to access ProtonVPN using an OpenVPN configuration file and VPN in Settings, I encountered a problem.
The instructions are fine as far as they go (NB the Router Config file from ProtonVPN seems to be the one to Download rather than a Linux one).
The issue I had was following the instructions in:
5. Go to “Account” in ProtonVPN for your credentials and copy them into the input boxes on your phone.
That sounds simple but there are two sets of IKEv2 credentials, The Username and the Password to enter. These are many characters long and Copy and Paste is the only way to reliably enter these.
Having copied and pasted one part of the credentials e.g. the username, you need to switch Apps to copy the second part (Password). This causes the dialogue box to close and the part you have just entered is forgotten!
This seems to be a major issue with this part of the Settings App design since the swiping Left to switch Apps shouldn’t close the dialogue, but does. It is therefore impossible to copy and Paste both the Username and Password so you can’t complete the setup that way.
The solution I found was to create an OpenVPN password file (a plaintext .txt file that just contains your ProtonVPN IKEv2 username and password) to which you point in the “Authentication Settings” part of the VPN Settings.
Having entered my credentials that way I seem to be connected to ProtonVPN.