Crosstab - Change over time

[Robl]

I have a crosstab.
It has 6 months on the top and a few measures as rows.
I need to show the change on one of those measures over the months.

Is there a simple way of doing this?
Data source is a simple star schema in SQL Server so I can't use any of the OLAP tricks.

For example.
If the numbers for Jan to April are;
10, 12, 14, 10

I need the change column to show
NA, 2, 2,-4


(My ways all involve convoluted methods involving second joined queries with month-1 calculated in there. Or making the report into a list and doing loads of calculation involving comparing dates).