Thanks for the reply - I wish the info about preferably not using a web hosts script had been in the documentation it might have save me a lot of time.

I am working on a clone of the site and am not using the web hosts script to upgrade. I have started with a new copy of 1.5.1 and am then customising as per the original version of the site, and then restoring the 1.3.9h db into this new version.

I don't want to start over as I have already spent about 12 hours on this project and still don't have a new version to test. So I will try changing the name of the admin folder and move on from there, unless anyone tells me that this won't work.