PHP Argument Code Concept Help Please

This is my node relationship model (relationships created using a nodereference field in the children content types);

Project
|
----> Job
|
----> Job
|
----> Job
|
----> Task
|
----> Task

My Goal:
Have a view in the Project Node that displays all the Tasks that are associated with it as evaluated by the downstream relationships Job -> Task. i.e. a double layer of relationships needs to be evaluated.

What I've done so far;
I've created a Calculated Field in CCK for each task to store the NID of the Project (evaluated by querying the Task's parent Job node reference field)

So now I have a Project (with an NID) and a list of tasks that have that same NID in a custom field - e.g. like a nodereference field, but not quite.

The bit I need help with:
I think that views arguments is the place where I want to be, but after that I'm not sure how to link the "Node ID from URL" back to the custom field. Maybe I need to be using the Provide Default Argument -> Default PHP code field?

I've looked at many examples but I can't see anything close to what I want and I can't conceptualise what to do from scratch.

Would greatly appreciate any assistance

Thanks,
Duncan