I have two taxonomies for categories of custom post type 'items'; 'type' and 'group'. They are not exclusive so one hierarchical taxonomy won't work although to an extent, as you'll see, they are hierarchical. So, for instance, I have a group 'group1' which may be in more than one type.

What I need to do is produce a taxonomy page 'type1' that list all the 'groups' with 'type1. It's easy enough to produce a list of posts in 'group1' but I don't know how to list the 'groups' that contain 'items' with type 'type1'.

I can see the logic but I'm struggling to produce the code. As these questions work better with some code, I'll update as I go.