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