Here is part of the code that I have written for the module

details.php

<?php defined('BASEPATH') or exit('No direct script access allowed');

/**
* This file is part of FireSale, a PHP based eCommerce system built for
* PyroCMS.
*
* Copyright (c) 2013 Moltin Ltd.
* http://github.com/firesale/firesale
*
* For the full copyright and license information, please view the LICENSE
* file that was distributed with this source code.
*
* @package firesale/prices
* @author FireSale <support@getfiresale.org>
* @copyright 2013 Moltin Ltd.
* @version master
* @link http://github.com/firesale/firesale
*
*/

class Module_Firesale_prices extends Module
{
    public $version = '0.1';
    public $language_file = 'firesale_prices/firesale';

    public function __construct()
    {
        parent::__construct();

        // Load in the FireSale library
        $this->load->library('firesale/firesale');
        $this->lang->load($this->language_file);
    }

    public function info()
    {

        $info = array(
            'name' => array(
                'en' => 'FireSale Prices',
                'fr' => 'FireSale Marques',
                'it' => 'FireSale Marche'
            ),
            'description' => array(
                'en' => 'Price Management',
                'fr' => 'Gestion des marques',
                'it' => 'Gestione Marche'
            ),
            'frontend'  => FALSE,
            'backend'   => TRUE,
            'author'    => 'Cameron Gilroy',
        );

        return $info;
    }

    public function install()
    {

        if ($this->firesale->is_installed()) {

            // Load requirements
            $this->load->driver('Streams');
            $this->load->model('firesale/products_m');
            $this->lang->load('firesale/firesale');

            #####################
            ## APPEND PRODUCTS ##
            #####################
            
            $template = array(
                'namespace' => 'firesale_products',
                'assign' => 'firesale_products',
                'type' => 'text',
                'title_column' => FALSE,
                'required' => TRUE,
                'unique' => FALSE
            );
            
            $fields[] = array(
                'name' => 'Church Price',
                'slug' => 'church_price',
                'type' => 'decimal',
                'instructions' => 'The price for churches',
                'extra' => array('decimal_places' => '2', 'min_value' => '0.00')
            );
            $fields[] = array(
                'name' => 'Church Price Tax',
                'slug' => 'church_price_tax',
                'type' => 'decimal',
                'extra' => array('decimal_places' => '2', 'min_value' => '0.00')
            );
            $fields[] = array(
                'name' => 'Reseller Price',
                'slug' => 'reseller_price',
                'type' => 'decimal',
                'instructions' => 'The price for resellers',
                'extra' => array('decimal_places' => '2','min_value' => '0.00')
            );
            $fields[] = array(
                'name' => 'Reseller Price Tax',
                'slug' => 'reseller_price_tax',
                'type' => 'decimal',
                'extra' => array('decimal_places' => '2','min_value' => '0.00')
            );
            
           // Add fields to stream
            foreach( $fields AS &$field ) { $field = array_merge($template, $field); }
            $this->streams->fields->add_fields($fields);
            
            return TRUE;
        }

    }

    public function uninstall()
    {

        // Load required items
        $this->load->driver('Streams');
        $this->load->model('firesale/categories_m');
        $this->load->model('firesale/products_m');

        // Remove prices field from products
        $this->streams->fields->delete_field('church_price', 'firesale_products');
        $this->streams->fields->delete_field('church_price_tax', 'firesale_products');
        $this->streams->fields->delete_field('reseller_price', 'firesale_products');
        $this->streams->fields->delete_field('reseller_price_tax', 'firesale_products');

        return TRUE;
    }

    public function upgrade($old_version)
    {
        return TRUE;
    }

    public function help()
    {
        return "Some Help Stuff";
    }

}

You've missed out combining $template and $fields before adding the fields to the stream hence the empty field namespace error. Just add the below:

foreach( $fields AS &$field ) {
     $field = array_merge($template, $field);
}

You can also just add all the key value pairs in $template to each of the $fields individually if you want (e.g. some fields might be required etc).

@AJSturrock I've posted the full file as I have it now - any suggestions? & it's still not working. :(

It's working now - lines 107-109 is where the problem was