When completing examinations candidates should be aware of the concession to round down VAT on an invoice.  More about this can be read on the online HMRC VAT Guide point 17.5.  This has been reproduced in full below.

Students asked to calculate the VAT from a gross figure should be aware that the concession to round down does not apply.

17.5 Calculation of VAT on invoices - rounding of amounts
Note: The concession in this paragraph to round down amounts of VAT is designed for invoice traders and applies only where the VAT charged to customers and the VAT paid to HM Revenue & Customs is the same. As a general rule, the concession to round down is not appropriate to retailers, who should see paragraph 17.6.

You may round down the total VAT payable on all goods and services shown on a VAT invoice to a whole penny. You can ignore any fraction of a penny.

Please note: HMRC VAT Guide note 17.5.1 explains that you can round (up or down) to the nearest whole penny or 0.5p if you wish to work out the VAT separately for each line of goods or services on an invoice.  The final amount of VAT payable can then be rounded down to the nearest whole penny.

For the purposes of ICB examinations VAT amounts have been callibrated so that you should not have any discrepencies here.