After much trial and error, this is what worked for me, and apologies for repeating basic stuff, but I also post this here for others.
Proton Mail → Proton Bridge → eM Client Configuration
First of all, download Proton Bridge and configure it to your existing Proton Mail account. The Proton Bridge download file is found under the Proton Mail settings button “IMAP/SMTP”, with options for Windows, macOS and GNU/Linux. Configuration is straight forward.
Run Proton Bridge, and note: Proton Bridge and eM Client both need to be always running to enable the configuration with eM Client to work and be ‘live’. It’s a good idea to use the ‘run on start up’ option on your PC.
Next in eM Client click top left ‘Menu’ -->Accounts–>+ Add Account.
On the next screen do not enter your email address in the box ‘Enter your email address and press start’). Instead, click the “Mail” option on this same page. This will bring up the next screen with a number of options Gmail, Outlook etc., choose “Other” and click next at the bottom of the screen.
On the next screen, you can now enter your email address. Important: this is the email address you use to sign in to your Proton Mail account, with the authentication being your Proton Mail password.
You now need to configure both IMAP and SMTP, to do this, first open Proton Bridge to access your mailbox details. These will show when highlighting the Proton Mail ‘Accounts’.
Copy the details (Proton Bridge has a handy ‘copy’ feature) and past them into the corresponding boxes in eM Client (yes, use the numbers 127.0.0.1 for ‘Hostname’). Change the Port.
In the box ‘Security Policy’ use drop down 'Force usage of SSL/TLS.
Next in ‘Authentication’ check the button ‘Use these credentials’ and input, “Username” (your Proton Mail email address) and now input that Proton Bridge generated password.
IMPORTANT: the ‘Username’ is the same as your Proton account email address, but the password is NOT. This is a special Proton Bridge generated configuration access password and must be input in the IMAP and SMTP setup screens only.
This is the same as for IMAP, just remember the Proton Bridge password as before.
You can then go to the “Diagnostics”, and run the ‘diagnose’ function to check if the configuration is correct.
If it all checks out then IMPORTANT, hit the “Save and Close” button at the top left, and you should be good to go.
Let me know if it works!