[frgsBC]

I seem to have worked it out, below is the necessary modifictaion if anybody else needs to do this.

File : include/func/func.cart.php around line 2700

Change from

  foreach($product_options as $o) {

                            $products_array['options_surcharge'] += (
                                $o['modifier_type'] == '%'
                                    ? ($products_array['price'] * $o['price_modifier'] / 100)
                                    : $o['price_modifier']
                            );
                        }

To this

foreach($products_array['product_options'] as $o) {
                            if($o['modifier_type'] != '%')
                                $products_array['options_surcharge'] +=$o['price_modifier'];
                            
                        }
        
                        foreach($products_array['product_options'] as $o) {
                            if($o['modifier_type'] == '%')
                            {
                                $products_array['options_surcharge'] += ($products_array['options_surcharge'] * $o['price_modifier']) / 100;
                                $products_array['options_surcharge'] += ($products_array['price'] * $o['price_modifier']) / 100;
                                
                            }
                        }

The change I have made is instead of doing all the modifiers at once, it now does the absolute modifiers first, followed by price modifiers.