I was able to import a bill.  The bill item had to exist in item maintenance.  I did not have UDF's in my source but my job looked like yours.  I think the issue you are having is how your source table is setup.  Details for header must be on every line.  Below is source file I used to import with 2 components.

MEMRY-00085,000,S,000,MEMORY CHIP SET - 1 MEG,,MCHIP-00256-80,36,
MEMRY-00085,000,S,000,MEMORY CHIP SET - 1 MEG,,ELECT-00001,2, 

I am having the same issue.  Did you happen to figure it out?  

Here are the snapshots of each relevant window in my case.  

i did try this, no luck for me. 

Yes, I found that I had to create the item in Item Maintenance First, before creating the BOM.

Strange, all of the Items i'm creating a BOM for are existing items already in our database, including the one in my example.  Maybe i'm doing something else wrong.  

Did you solve this IanD? I'm trying to import ScrapPercent to my existing BOMs as well. Edit: I was able to import Scrap Percent by using BillNo, Revision, Line Key, Scrap Percent. All good!