Ok my mistake, you can see my images array was not quite right:
This is correct:
'images' => [
[
'src' => 'http://demo.woothemes.com/woocommerce/wp-content/uploads/sites/56/2013/06/T_2_front.jpg'
],
[
'src' => 'http://demo.woothemes.com/woocommerce/wp-content/uploads/sites/56/2013/06/T_2_back.jpg'
]
],
This was mine:
'images' => [
'src' => 'http://demo.woothemes.com/woocommerce/wp-content/uploads/sites/56/2013/06/T_2_back.jpg',
],
Thought it would be best to leave this question/answer as over the years there are many questions about this online with the same error.