Power Query M add previous value column to grouped data

Hi,
I've been trying to add a column to grouped 'all rows' which give previous row value;

I have this working in non group ;

Table.AddColumn(#"Changed Type", "Custom", each List.Last( 
                                                                                 Table.SelectRows( #"Changed Type" , (A)=>   A[Date] < _ [Date] and A[Item] = _ [Item]
                                                                                                                                                                                                              )[Unit] ) ))

But I thought it might be easier to group by iitem and do the same for each group;

Table.Group(#"Changed Type", {"Item"}, {{"Count", each Table.AddColumn( _ , "new", (B)=> 1)}} )

and then tried to build on that,

Table.Group(#"Changed Type", {"Item"}, {{"Count", each Table.AddColumn( _ , "new", (B)=>    Table.SelectRows( _  , (A)=> A[Date] < _ [Date] ) ) }} )

I've managed to make a function and invoke on the grouped date shown in attachment,

Richard