Does file_no have a unique constraint on it in table dbo.DebtorInfo?

Does the form just update one row or does it change multiple rows? i.e. how many rows are in the inserted table?

drop table #Table;
go
create table #Table
(
    ProductLine [nvarchar](2) NULL,
    BusinessUnit [nvarchar](20) NULL,
)

insert into #Table(ProductLine,BusinessUnit )
values('1Y',''),
('1Y', 'IT Product'),

Just write a small block of code for each of your 10 tables:
DECLARE @BatchSize int = 100000
DECLARE @RowsDeleted int = @BatchSize 
WHILE @RowsDeleted <> @BatchSize BEGIN

When you have a query that isn't returning enough results a good way to diagnose it is to remove constraints on the query until you get some rows back.
So...

;WITH myTable as
(
SELECT *
  FROM (VALUES (1001, 'X'),
               (1002, 'YY'),
             ...

If you set your variable to a csv string e.g.:
DECLARE @Subjects as nvarchar(100) = 'economics,maths'
Then use a string splitter function to convert search value into a...