Sales Tax recording

[Gerard]

When I generate a sales invoice or a purchase invoice, I add VAT @ 21% to the nett total
    When I post to Accounts Receivable and Sales Account, these accounts record the full
    sales and purchases amounts incl of Vat. I don't understand the process of recording or 
splitting nett amounts from the Tax amounts, How do I set up separate accts to record nett sales
    and a separate Vat account for Sales & purchases. I thought GnuCash did this automatically.
    I went thro' the help tutorials but can't find what I am looking for
Looking forward to some assistance, 

Gerard