drill through from heat map not passing value

[Blue]

G'day all

CA 11.0.4 on Linux (Red Hat) using DB2 10.5 data source.
Reports using Full interactive mode.

Report A contains a heat map. This has the dimensions Model Name and Unit Code and the fact Health Score.
When a user clicks a cell in the heat map a second report (B) should open in a new tab/window to display details for the selected Unit-Model combination. A model name could be "Transformer" (code = 'AAA') and a Unit code "XXX01".

Report B has three prompts. It can be accessed either directly from the Welcome page or via one of two drill throughs.
Prompt 1 is for Station Name (parent of a Unit), Model Name, and Component (Unit Model code). The first two prompts are used to filter the list displayed in the 3rd. A Station Name has the format "Xxxxxx", e.g. "Wellington" (code like "XXX"). A Model Name same as above. A Component has the form "XXX01/AAA" where XXX01 is the Unit Code as above and AAA id the Model Code.

When a user does a DT from Report A to Report B the Station Name and the Model Name get passed to the prompts just fine. But the Unit Model Code does not! The former comes from a prompt response on Report A and the latter from the Heat Map viz on Report A.  But while the Unit Model Code is in the query that drives the Heat Map it isn't in the Data Container (as it is the combination of Unit code and Model Code).

I tried using a hidden prompt on Report B to get the Unit code and then build the Unit Model code dynamically - didn't work.

I've attached a Word doc showing the settings.

Has any of you seen this problem before? Did you find a work-around?

Cheers,

Blue