thanks for the indepth Armando!
I think the parents way is the best too at this stage
Actually,
I could in my "Files" grid make an invoice parent and then, in "WorkDone" use ID number for invoice related work*
So for each invoice the WorkDone will be available in a sortable field, and,
the Files worked on will be separately available on a per invoice basis in the "Files" grid
PS you've given me a few good tips on searching & other stuff there

EDIT/
* in "WorkDone" I have files as Parent items but wont give them InvoiceIDs themselves
i.e. File has subitems that are given invoice number
-
SQLNotes01.psd- -
Drawing of same - 2hrs - Invoice#200
EDIT2/
then in files grid I have now added the invoice (as a file/link)
and copied the relevant files to it
-
Invoice200.doc - 4hrs - $2xyz
- -
SQLNotes01.psd - 2hrs - $xyz
- -
SQLNotes02.psd - 2hrs - $xyz
EDIT2/
under Invoices grid, using InvoiceID as source (based on first edit there) and if I show the "ItemParent" field
I should see
-
InvoiceID #200 - 4hrs - $2xyz
- - Drawing -
SQLNotes01.psd - 2hrs - $xyz
- - Drawing -
SQLNotes02.psd - 2hrs - $xyz
so I have it doubly covered really

Will go with this for a while and see how I get on