Using weekly data to convert to monthly

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I already have quite a comprehensive spreadsheet (imho) thanks to the guys on here and I would like to use the data I have which is broken into weeks converting into months.

So, I have a column with all the weeks with week commencing dates. On another sheet I would like to consolidate the week commencing rows into a monthly figure. For example, 05/07/10, 12/07/10, 19/07/10 and 26/07/10 would be combined into a single July row.

I have attached the spreadsheet which should show it clearer. I thought at first it would be a case of pointing the 4 cell references (4 weeks) to the monthly sheet but some months fall with 5 week commences.

Many Thanks
David