Unattended Access public key error
Based on your comment above, when I create a new key and copy and paste it to a file (instead of using the save to a file option) the key is now valid and can be used to gain access to the object computer.
After that I found another issue. The object computer is a recent build and was configured with a Microsoft account and PIN to sign in. Remote access was being rejected with invalid credentials. In desperation, I changed the login to a local account and a traditional password - the remote access is now working.
Thanks for your help, this issue is now resolved in my case.