APEX

Good afternoon.
I'm not sure if this is even possible but hopefully one of you experts out there will be able to advise;
I had some javascript in my page which contained a simple function to open a BI Publisher report, it was:

function run_40BFG()
{
popupURL('&BI_URL.Guest/MyFolder/MM_40BFG/MM_40BFG.xdo?_xpf=&_xpt=1&P_CASE_ID=&P303_CASE_ID.&xt=MM_40BFG_xf=rtf');
}

...this worked fine. To improve maintainability (I don't want to amend the Page Header each time I add a report), I wanted to extract this function and place it in a javascript file which I have done with 'some' success.

Page Template amended - OK
Page Header global variables added - OK (tested with simple 'alert' function and all was fine)

Now the function in the javascript file looks like:

function run_40BFG()
{
popupURL('gBIurl.Guest/MyFolder/MM_40BFG/MM_40BFG.xdo?_xpf=&_xpt=1&P_CASE_ID=gCaseID.&xt=MM_40BFG_xf=rtf');
}

...but each time I receive a 500 Internal Server Error.
Is it at all possible to run a BIP report from an external javascript file? If not, is there another way?
If it is possible, what am I doing wrong?

Thanks in advance!
Sue