Show subnav menu on specific pages

Hi,

I'm trying to create a structure as follows:

ContainerA
--CategoryA1
----CategoryA1a
------CategoryA1a1
------CategoryA1a2
----CategoryA1b
------CategoryA1b1
------CategoryA1b2
----CategoryA1c
----ContainerB
------CategoryB1
------CategoryB2
--CategoryA2
----etc.

It's pretty straightforward to get the items in ContainerA to show up as primary, secondary and tertiary menus, but how can I get the categories in ContainerB to show up as a separate menu block, but only when viewing nodes belonging to CategoryA1 and not for nodes belonging to CategoryA1's subcategories? I assume I will need to add some PHP code to get the relevant block to show up on nodes belonging to specific categories (and not to others) but I'm not sure where to start with this.