Ok so rather than getting SKUs for each product I have found it easier to get ALL the SKUs separately.
Firstly I found out how many SKUs are available in total by CURLing the following URL.
/api/v2/products/skus/count.json
And the same for the parent products
/api/v2/products/count.json
The divided these by 100 (as I would page them by 100)
I then used the following commands in the PHP API provided by bigcommerce For SKUs..
Bigcommerce::getSkus($filter)
For Products...
Bigcommerce::getProducts($filter)
I then used the following filter.
$filter = array('limit' => 100, 'page' => $AmountOfPagesReturnedFromCurl);
This will then loop it through all the SKUs/Products which I can add SQL commands in the loop to add them to my DB.
I hope this is helpful to someone, I really enjoy developing with Bigcommerce.