I'm the author of the Excel spreadsheet and would second umop3p!sdn in suggesting that you should use 'official' software for the purpose of calculating something as fundamental as a pay amount. The consequences of errors in this field can be pretty serious to deal with!
While I am happy with the information the spreadsheet gives (it tends to accurately reflect the pay calculations of those whom I have tested it against), I do not offer any warranty that the spreadsheet always accounts correctly for every conceivable pay related income or deduction- it's not designed as a 'warranted product'. Also, it doesn't produce payslips/ P35's etc