Hi Howard
Many thanks for your reply.
After much reading and testing with no success, I tried starting from scratch with new backups of my original site DB, moodle files and moodledata folder and did get successful migration and upgrade - at least without the repeat login issues. (The second time I was able to run the replace.php script and this seemed to fix the repeat login issue.)
I now have a different problem, in that from 2.2.11, site images are missing.
As I have not used images extensively, I could just add them again in 2.2.11 but wonder if it is how I upgraded which caused the issue?
An outline of what I did is below:
For making the copy installation:
I made the copy by backing up the DB, and compressing and downloading the moodle files and moodledata folders first.
Then created a new DB and imported my saved copy into it.
Then unzipped the original moodledata copy folder to a new, empty folder - moodledata2 (then moved all files so directly in the moodledata2 folder.)
Then repeated the above with the moodle folder files - moodlecopy.
Then I changed config.php in the moodlecopy folder to reflect the new db name, URL and moodledata2 locations/names/password as appropriate.
For the upgrade:
To 1.9.19 - I changed the name of the moodlecopy folder to moodlecopy-old
Created a new empty folder called moodlecopy and uploaded and extracted the 1.9.19 installation zip downloaded from Moodle site.
Then copied across the config.php file from the moodlecopy-old folder.
Then logged in as Admin and ran the upgrade.
I repeated this process without any errors for 2.2.11 and then 2.6 - have stopped there for now.
Could this process by why images are missing?