Displaying prices inclusive of VAT

[tsrplatelayer]

In my shop http://www.freight-stop-shop.co.uk/driving/ I have set the prices to display as 'Gross' i.e. including VAT.

This means that on the checkout-payment page the subtotal dont 'add-up' i.e. the sub total is shown as the 'gross' price, shipping is listed (0 in ths case) and then VAT is shown, Finally the total is shown - which is not the sum ofe the above entries.

How can I display the price as inclusive of VAT on the products listings and on the checkout-payment page show the subtotal without VAT included.

Advice greatly appreciated

[tsrplatelayer]

I can see in includes/classes/order.php line 513 that the tax content of tax-included prices is calculated, however I cannot see that shown-price is changed to reflect tax-excluded price.

      if (DISPLAY_PRICE_WITH_TAX == 'true') {
        // calculate the amount of tax "inc"luded in price (used if tax-in pricing is enabled)
        $tax_add = $shown_price - ($shown_price / (($products_tax < 10) ? "1.0" . str_replace('.', '', $products_tax) : "1." . str_replace('.', '', $products_tax)));
      } else { 

I ask becaue PayPal reports an error that sub totals dont add up