For safety's sake, I'd suggest a unique composite index, just to make sure
that the InvoiceNumber - FPNumber combination is unique.
This can be the primary key of your InvoiceDetails table.
It should be the primary key of your FurtherInformation table.
You can then define a one-to-one relationship between the two tables, with a
cascade update from InvoiceDetails to FurtherInformation. (This will not by
iteself require a record in FurtherInformation for each record in
InvoiceDetails.)
Now you can create a query linking both tables, with a join which says "show
all records for InvoiceDetail and only related records for
FurtherInformation." Base your "new" form on this query; I'll call it
frmFurther, but you can name it as you like.
In whatever event procedure you want to use to show this new form, use code
like this:
DoCmd.OpenForm "frmFurther",,,"[Invoice Number] = " & [Invoice Number] &
" AND [FP Number]=" & [FP Number]
I think that will do it. If you're launching the new form from the main
form, you'll need to qualify the control references.