Pivot Table: Group by month splits weekly results

Hi all,

I have a set of raw data (that I can't change at source) which I have tried to group by year, month and week using a pivot table. So far so good.
I am using an index/match formula on a different sheet to lookup values on that pivot table (formula looks out for week vertically and both column labels horizontally, e.g. LC_RT_1&"Total Sends" and then returns the value from the cell it can find). Problem now is that the pivot seems to split the grouped weeks as a few days of some weeks fall into two months. E.g. week48 falls into Month 11 (M11) and Month 12 (M12) - see attached screenshot and example file.

As the formula on the dashboard would only show the first value that it can find, it basically ignores the results from the 2nd time the same week shows up in the data.
Is there any way I can change the pivot table to only show a week once and, for example, attribute the results of that week to either of the months (e.g. combine week48 results and attribute to M11 or M12)?
Hope you understand what I mean All a bit complicated!

Hope you can help.
Thanks!