Are you using SAGE if so - print off your sales nominal activity for these dates to check if your sales figures are correct - I would also run off your detailed VAT return for this period also.
Also check the dates of invoices posted, ie have sales invoices for an earlier period been posted with a default (and later) date? However if this is the case you may have under declared the previous quarter.
if not using sage, what are you using. do you have a sales daybook?