SSRS matrix with Previous function

Hi all

I have a matrix that measures progress by month of product sales. I would like to display the difference between months e.g. gone up, down, stayed the same and have tried using the Previous function but it errors (the use of a Previous aggregate function in a tablixcell within tablix table is not supported).

The report is built on fetchxml so there's nothing funky I can do in sql to help and I have no choice about that. I need to end up with something that looks like this:

                Jan                                Feb                                          Mar
               No.    Value      Diff         No.      Value    Diff        No.     Value     Diff

Bikes 3 500 2 400 -100 5 700 300
Cars 7 100 10 500 400 5 250 -150

I hope someone can help!

Many thanks