If you have an invoice with multiple lines on it and you then check the VAT amount is the correct percentage of the Net amount, you may find it to be a few pennies different from what you'd expect.  
This is because VAT is calculated on each line and rounded to two decimal places.  Then at the bottom of the invoice the sum of these calculations is used as the total VAT due.  
When you have multiple lines on an invoice the sum of these rounded figures can sometimes be different too if you add up the total of the Net figures for each line and calculate the relevant percentage of that.  
This is expected and correct as per Section 17.5 of the HMRC VAT Guide.

Here is an example:

Let’s say a Hirer books 10 sessions at £11.31

The booking confirmation calculates: 
£11.31 x 10 = £113.10
Then adds 20% VAT = £22.62.  
The Total is £135.72

The invoice calculates each line separately:
£11.31 + £2.26 (actually £2.262 but rounded down)
The Total is £135.70

So the effect is a 2p discrepancy between the two figures across the full 10 sessions.

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