Thanks for your post.

This issue has been added to the tracker:
https://winscp.net/tracker/1468

Meanwhile, just add your .ppk file to WinSCP session configuration:
https://winscp.net/eng/docs/ui_login_authentication

I installed WinSCP, it offered to copy settings from FileZilla. Presumably it did not copy my existing pkk as connection failed.

This tutorial explains how to enter a pem which will be converted.
https://winscp.net/eng/docs/guide_amazon_ec2

Rather than trying to find the original pem is there a way to add the working pkk file that I already have?

Thanks ColinK