Thanks for the pointer, Jerome.
I have had a go and achieved a kind of result (!) - see attached image using the following code:
---
/*ken*/
<tr>
<?php echo $td; ?>
<td id="hikashop_checkout_cart_total2_title" class="hikashop_cart_subtotal_title hikashop_cart_title">
<?php echo JText::_('HIKASHOP_TOTAL'); ?>
</td>
<td class="hikashop_cart_subtotal_value">
<?php
$this->row = $this->total;
echo $this->loadTemplate();
?>
</td>
</tr>
/*ken*/
---
inserted as below.
Query 1: how can I do the sum in PHP?
Query 2: Woud making such a change (adding sub-tal) impact on code lower down the cart (e.g. coupons etc if they exist)?
Thanks for taking a look.
Ken 
---
Code extract from cart:
---
<tr>
<?php echo $td; ?>
<td id="hikashop_checkout_cart_shipping_title" class="hikashop_cart_shipping_title hikashop_cart_title">
<?php echo JText::_('HIKASHOP_SHIPPING'); ?>
</td>
<td class="hikashop_cart_shipping_value" >
<span class="hikashop_checkout_cart_shipping">
<?php
if(isset($this->value)) {
echo $this->value;
} else {
$shipping_price = null;
foreach($this->shipping as $shipping) {
if(!isset($shipping->shipping_price) && isset($shipping->shipping_price_with_tax) ) {
$shipping->shipping_price = $shipping->shipping_price_with_tax;
}
if(isset($shipping->shipping_price)) {
if($shipping_price === null)
$shipping_price = 0.0;
if(bccomp($taxes,0,5)==0 || !$this->params->get('price_with_tax') || !isset($shipping->shipping_price_with_tax)) {
$shipping_price += $shipping->shipping_price;
} else {
$shipping_price += $shipping->shipping_price_with_tax;
}
}
}
if($shipping_price !== null) {
echo $this->currencyHelper->format($shipping_price, hikashop_getCurrency());
}
}
?>
</span>
</td>
</tr>
/*ken*/
<tr>
<?php echo $td; ?>
<td id="hikashop_checkout_cart_total2_title" class="hikashop_cart_subtotal_title hikashop_cart_title">
<?php echo JText::_('HIKASHOP_TOTAL'); ?>
</td>
<td class="hikashop_cart_subtotal_value">
<?php
$this->row = $this->total;
echo $this->loadTemplate();
?>
</td>
</tr>
/*ken*/
<?php
}
if(!empty($this->additional)) {
$exclude_additionnal = explode(',', $this->config->get('order_additional_hide', ''));
foreach($this->additional as $k => $additional) {
if(in_array($additional->name, $exclude_additionnal))
continue;
?>
<tr id="hikashop_checkout_cart_additional_<?php echo str_replace(' ','_',$k); ?>_line" >
<?php echo $td; ?>
<td id="hikashop_checkout_cart_additional_<?php echo str_replace(' ','_',$k); ?>_title" class="hikashop_cart_additional_title hikashop_cart_title"><?php
echo JText::_($additional->name);
?></td>
<td class="hikashop_cart_shipping_value" >
<span class="hikashop_checkout_cart_shipping">
<?php
if(!empty($additional->price_value) || empty($additional->value)) {
if(bccomp($taxes,0,5)==0 || !$this->params->get('price_with_tax')){
echo $this->currencyHelper->format(@$additional->price_value,$additional->price_currency_id);
}else{
echo $this->currencyHelper->format(@$additional->price_value_with_tax,$additional->price_currency_id);
}
} else {
echo $additional->value;
}
?>
</span>
</td>
</tr>