Auto filling blank cells with a number...

It might be a simple problem which my brain is just too small to cope with... but:

I have a very long column of 6 figure numbers with some groups of cells left blank. These blank cells are really the value of the number in the cell above. However blanking was caused as result of using the pivot table.

I've managed to get to a point where I can read the cell above by creating a new column and putting the following IF statment in: =IF(A2=0,A1,A2)
This works great, but when there's more than one blank cell in the column... yeiks, I obviously end up with another 0 in the cell, not the number a couple cells above.

Does anyone know how to auto fill downwards blank cells?