If you've missed the September date and need to calculate your income tax youself, or you want to check the Inlands Revenue's calculation you will find this spreadsheet useful. The Tax99 spreadsheet is set out as far as possible to look like the tax return itself. This archive should contain four files:- Readme.txt - This file Info - Detailed instructions on using the spreadsheet Tax99 - The tax return spreadsheet SE99 - The supplementary Self-Employment spreadsheet.