Hello Iain,
If you are using Moodle 2.6, then Atto is not part of the core. I assume you or an admin downloaded Atto from the plugin
database. The version for 2.5 and 2.6 in the database is a working prototype of Atto as it was being developed to allow users to evaluate it. The version in 2.7 has better table support. The older versions have some differences that make back porting of features difficult. I do not think that they have been updated in the last six months. In 2.6, TinyMCE is still the supported core editor. If most everyone is happy with Atto, those that need better table support can override the default in their profiles and use TinyMCE. Otherwise, you may upgrade to Moodle 2.7.
