Thanks for your explanation, which, now, makes perfect sense and was the cause of my problem.
I have a mix of files encrypted with Version 1 and Version 2. Both had different passwords and is the root of the problem.
When I signed up for version 2 I thought it best to use a better password. Naively I thought Ver.2 would decrypt the old files, obviously after you explanation it makes perfect sense that it wouldn’t as the passwords are different.
Putting my old password into the second dialogue box wouldn’t work so I uninstalled Ver 2, installed Ver 1 and decrypted all my old files. I’ve now reinstalled Ver 2 and will encryt all my files with this.
Got to thank you for your assistance in solving my, self inflicted, problem.