When you install Sage Line 50, the T9 tax code is set as the default code for transactions which do not involve VAT and are therefore not to be included on the VAT Return. This is used for all the routines within Sage Line 50 that are non-vatable, for example the year end journal entries and error corrections.
If you want to change this default tax code, open the Settings menu, choose Tax Codes, select T9 from the Tax Codes list and click Edit. You can now edit the settings.

Note: If you change the function of the T9 tax code, you must create another non-vatable tax code and set it to be the default non-vatable tax code. To amend the non-vatable tax code from the Settings menu, choose Company Preferences, open the Parameters tab and change the default to your new tax code.

The following settings should be correct before you start processing your data:
Financial year start date.
Vat scheme.
Control Accounts.

This is because you have posted transactions onto the audit trail. If you have only entered a limited number of transactions, you can change the financial year date by using the Rebuild New Data from the Maintenance option on the File menu. At this point, retain all the data except your transactions. Then you will be prompted to enter your correct financial year start date.

You would change the program date for the period end routines, such as month and year end procedures.
Also, if you are entering a large amount of data for a specific date it may be easier to change the program date rather than change the date on each individual transaction.