For Flat rate VAT you can just multiply the turnover by the rate for the period.
Assuming everything entered into sales should be included in the "flat rate turnover" (see http://www.hmrc.gov.uk/vat/start/sch...at-rate.htm#4)
, you can calculate the VAT due by multiplying the total gross sales for each month by the VAT rate in force in that month. (Cell F1 in Sales.xls for each month).
So for a VAT return covering December 2009 - February 2010, using the rates quoted above this would be:
Sales.xls > Dec09 > Cell F1 * 9.5% +
Sales.xls > Jan10 > Cell F1 * 11.5% +
Sales.xls > Feb10 > Cell F1 * 11.5%