Thank you for your feedback. Since this appears to be specific to my setup I will document as best I can exactly what I am doing:
I am running Linux (Centos 4)
To get ready for a clean install I Drop The Database, Recreate The Database and Clear Root Directory
I then download magento-0.6.13700.tar.gz (light) and then un-tar it using the -xzvf parameters
On visiting http://domain.com I am presented with the installation process.
I install Magento without error and set my locale to UK, timezone to Europe/London and currency to British Pounds Sterling
The very first thing I do after installation is go to the back-end, login and go to Catalog -> Attributes -> Manage Attribute Sets
I then select “Default” which is the only Attribute Set available after a clean install
The four errors (in order) are:
Invalid property - js/extjs/ext-tree.js (line 62)
Ext is not defined - js/extjs/ext-tree-checkbox.js (line 5)
Ext is not defined - admin/catalog_product_set/edit/id/9/ (line 745)
Ext is not defined - admin/catalog_product_set/edit/id/9/ (line 53)
It appears that the second, third and fourth error are only caused as a result of the first (invalid property) error.
As I have already mentioned, editing attribute sets was working fine with the previous Magento release so this appears to be specific to 0.6.13700