Ok, so found a strange “feature” with SharePoint 2010.
The scenario goes like this:
You have a new Intranet, setup your Intranet Owners (who have full control from Site Collection Level down) and created some new SharePoint groups for content managers.
You then give the Content Managers groups Read Access to the top site collection and the Design permission level to the relevant pages (or folder in the Pages library) that you want them to be able to edit. (incidentally, a major change from SharePoint 2007 here – you can now only have ONE Pages library in a publishing site).
You then find the users in those groups cannot actually edit their page content or add web parts.
What you need to do is create a new Permission Level at the top site collection, that only has “Add and Customise Pages” (plus the other required permissions that get added when you enable that one).
Then give the Content Managers groups this Permission Level to the Top Site Collection. You can also restrict their permissions to the pages you do not want them to access in the Pages library.