Excel 2007 : Pivot table % of previous month query

I have a pivot table based on sales by month with columns as values and rows as dates which are grouped by months and years

values are sum of sales and i have added another sum of sales column which i want to show the % growth or dep per month

i have set the value field settings to % diff from with base field as month and base item as previous

this is now working fine apart from Jan which isnt picking up the % diff from Dec - if i remove the year filter it then totals the months from each year

i have attached simplified version of the pivot table in case i havent explained it very well

i've done a bit (lot) of a bodge job to kind of fix it temporarily - had to create a seperate table next to the pivot which feeds from the pivot and enter a manual formula to calculate Jan 2010/Dec2009

Although not ideal it could be worked around as going forward the data will always be the last full month - 1 year

I'd still be grateful if anyone knows if there are some pivot settings i can change to get this working properly

(also having fun with the formatting the chart to go with it but thats a question for another section.....)