Statement of account from rows of Excel spreadsheet

I'm trying to produce a number of letters (or emails) in Word 2007 using Mergings each letter/email with a unique account number.
I have a Word template and an Excel spreadsheet. The spreasheet has a list of invoices.

(I've attached a sample of each. The cell highlights are not needed on the table - they are only there to highlight the changes of account number)

I want to mailmerge the first row(s) of data in Excel to a table in the Word document keying on the account number. When the Account number in the record changes I want the table to finish and Word to produce the next statement keying on the next account number.
What is the conditional formatting needed to read each row of data across to the first Word letter and change when the account number changes?
Thanks for any help you can give. Steve